Can't see the chat messages on Yahoo Messenger.

Here's the solution when you can't see the messages you send on yahoo messenger and cannot see the messages received.

Just follow this simple two steps.

Step 1

•  type on run regsrv32 jscript.dll then press OK.

Step 2

• type on run regsrv32 vbscript.dll then press OK.

Command Reference for IPCONFIG MS-DOS Command

Ipconfig is a MS-DOS command-line tool used to display and manage the network settings of your computer. Ipconfig is available on Windows machines, and it displays the current network connection details and DHCP client settings.

Ipconfig is an external MS-DOS command, and is available on Windows 95, Windows 98, ME, NT, 2000, XP and Windows Vista Operating Systems. On Windows 9x machines, a graphical tools such as "winipconfig" or "winipcfg" may be used instead. On Linux machine, the ifconfig command performs equivalent function.

C:\> ipconfig /?

USAGE:
    ipconfig [/? | /all | /renew [adapter] | /release [adapter] |
              /flushdns | /displaydns | /registerdns |
              /showclassid adapter |
              /setclassid adapter [classid] ]

where
    adapter         Connection name
                   (wildcard characters * and ? allowed, see examples)

    Options:
       /?           Display this help message
       /all         Display full configuration information.
       /release     Release the IP address for the specified adapter.
       /renew       Renew the IP address for the specified adapter.
       /flushdns    Purges the DNS Resolver cache.
       /registerdns Refreshes all DHCP leases and re-registers DNS names
       /displaydns  Display the contents of the DNS Resolver Cache.
       /showclassid Displays all the dhcp class IDs allowed for adapter.
       /setclassid  Modifies the dhcp class id.

The default is to display only the IP address, subnet mask and
default gateway for each adapter bound to TCP/IP.

For Release and Renew, if no adapter name is specified, then the IP address
leases for all adapters bound to TCP/IP will be released or renewed.

For Setclassid, if no ClassId is specified, then the ClassId is removed.

Examples:
    > ipconfig                   ... Show information.
    > ipconfig /all              ... Show detailed information
    > ipconfig /renew            ... renew all adapters
    > ipconfig /renew EL*        ... renew any connection that has its
                                     name starting with EL
    > ipconfig /release *Con*    ... release all matching connections,
                                     eg. "Local Area Connection 1" or
                                         "Local Area Connection 2"

C:\>ipconfig /all

Windows IP Configuration

        Host Name . . . . . . . . . . . . : topwebhosts
        Primary Dns Suffix  . . . . . . . :
        Node Type . . . . . . . . . . . . : Hybrid
        IP Routing Enabled. . . . . . . . : No
        WINS Proxy Enabled. . . . . . . . : No

Ethernet adapter Wireless Network Connection:

        Connection-specific DNS Suffix  . :
        Description . . . . . . . . . . . : Intel(R) PRO/Wireless LAN 2100 3B Mi
ni PCI Adapter
        Physical Address. . . . . . . . . : 00-0C-F1-65-5B-70
        Dhcp Enabled. . . . . . . . . . . : Yes
        Autoconfiguration Enabled . . . . : Yes
        IP Address. . . . . . . . . . . . : 192.168.1.100
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 192.168.1.1
        DHCP Server . . . . . . . . . . . : 192.168.1.1
        DNS Servers . . . . . . . . . . . : 192.168.1.1
        Lease Obtained. . . . . . . . . . : Thursday, February 08, 2007 2:27:17
PM
        Lease Expires . . . . . . . . . . : Thursday, February 15, 2007 2:27:17
PM

Ethernet adapter Local Area Connection:

        Media State . . . . . . . . . . . : Media disconnected
        Description . . . . . . . . . . . : Intel(R) PRO/1000 MT Mobile Connecti
on
        Physical Address. . . . . . . . . : 00-0D-60-FB-4E-E9

To learn the names of ethernet adapters that you can optionally specify with "ipconfig" command, you may simply type ipconfig command by itself. The command output displays all adapters by name that are available on your computer: e.g. "Local Area Connection", "Wireless Network Connection".

Ipconfig command is most often used to diagnose network problem on a Windows machine. If you're using DHCP, you may try releasing and renewing IP address by performing "ipconfig /release" and "ipconfig /renew" commands shown below.

C:\> ipconfig /release
Windows IP Configuration

No operation can be performed on Local Area Connection while it has its media di
sconnected.

Ethernet adapter Wireless Network Connection:

        Connection-specific DNS Suffix  . :
        IP Address. . . . . . . . . . . . : 0.0.0.0
        Subnet Mask . . . . . . . . . . . : 0.0.0.0
        Default Gateway . . . . . . . . . :

Ethernet adapter Local Area Connection:

        Media State . . . . . . . . . . . : Media disconnected

C:\> ipconfig /renew

Windows IP Configuration

Ethernet adapter Wireless Network Connection:

        Connection-specific DNS Suffix  . :
        IP Address. . . . . . . . . . . . : 192.168.1.100
        Subnet Mask . . . . . . . . . . . : 255.255.255.0
        Default Gateway . . . . . . . . . : 192.168.1.1

To workaround DNS caching issue, you may perform "ipconfig /flushdns" to clear DNS cache value on your computer. DNS uses TTL (Time-To-Live) value which let the intermediate name servers to cache DNS information. If you changed your DNS settings, and your computer doesn't see the change immediately, you may perform "ipconfig /flushdns" to clear the DNS cache.

C:\> ipconfig /flushdns

Windows IP Configuration

Successfully flushed the DNS Resolver Cache.

FAQ:

On Windows Vista machine, if you are getting an error "The requested operation requires elevation", you'll need to run Command Prompt as an administrator. To do this, you'll need to do the following:

All Programs -> Accessories -> right click Command Prompt and click Run as administrator!

What is Denial of Service (DoS) attack?

DoS attack, denial-of-service attack, is an explicit attempt to make a computer resource unavailable by either injecting a computer virus or flooding the network with useless traffic. There are two types of DoS attacks: computer attack and network attack. Common forms of denial os services attacks are:

Ping of death 

Ping of death is caused by an attacker deliverately sending a ping packet, normally 64 bytes, that is larger than the 65,535 bytes. Many computer systems cannot handle an IP packet larger than the maximum IP packet size of 65,535, and often causes computer systems crash. It is illegal to send aping packet of size greater than 65,535, but a packet of such size can be sent if it is fragmented. When a receiving computer reassembles the packet, a buffer overflow occurs, which often causes computer to crash. This exploit has affected a wide variety of systems including Unix, Linux, Mac, Windows and routers; but the fixes have been applied since 1997 making this exploit mostly historical.

Ping of flood 

Ping of flood is caused by an attacker overwhelming the victim's network with ICMP Echo Request (ping) packets. This is a fairly easy attack to perform without extensive network knowledge as manyping utilities support this operation. A flood of ping traffic can consume singificant bandwidth on low to mid-speed networks bringing down a network to a crawl.

Smurf Attack 

Smurf attach exploits the target by sending repeated ping request to broadcast address of the target network. The ping request packet often uses forged IP address (return address), which is the target site that is to receive the denial of service attack. The result will be lots of ping replies flooding back to the innocent, spoofed host. If number of hosts replying to the ping request is large enough, the network will no longer be able to receive real traffic.

SYN Floods 

When establishing a session between TCP client and server, a hand-shaking message exchange occurs betwen a server and client. A session setup packet contains a SYN field that identifies the sequence in the message exchange. An attacker may send a flood of connection request and do not respond to the replies, which leaves the request packets in the buffer so that legitimate connection request can't be accommodated.

Teardrop Attack 

Teardrop attack exploits by sending IP fragment packets that are difficult to reassemble. A fragment packet identifies an offset that is used to assemble the entire packet to be reassembled by the receiving system. In the teardrop attack, the attacker's IP puts a confusing offset value in the sebsequent fragments and if the receiving system doesn't know how to handle such situation, it may cause the system to crash.

Mail Bomb 

Unauthorized users send large number of email messages with large attachments to a particular mail server, filling up disk space resulting in denied email services to other users.

What is distributed DoS (DDoS) attack? 

DDoS (Distributed Denial Of Service) is a tactic used to attack a victim from multiple compromised computers. Attacker installs a virus or trojan software on compromised systems, and use them to flood a victim's network in a way that the victim's server cannot handle it.

DDoS involves 3 parties: an offender, helpers and a victim. The offender is the one who plots the attack, and helpers are the machines that are compromised by the offender to launch attack against a victim (the target). The offender commands the helpers to attack the victim's host at the precisely same time. Due to this co-ordinated nature between the offender and helpers, the DDoS is also known as co-ordinated attack.

Resolutions 

If you suspect a DoS or DDoS attack due to a significant network slowdown or denied service, you may execute a few diagnostic Linux commands to find a host under attack.

First, you'll have to identify a host under DoS or DDoS attack. To do this, you'll have to monitor network traffic and see where the traffic is coming from and where they are going. This can be done with ethereal or tethereal Linux command.

# tethereal
  0.809751  10.1.1.5 -> 192.168.1.4 IP Fragmented IP protocol 
(proto=UDP 0x11, off=2960)
  0.810357  10.1.1.5 -> 192.168.1.4 IP Fragmented IP protocol 
(proto=UDP 0x11, off=1480)
...

## If you do not have ethereal installed, you may use 'yum' to
## install it on your system.
# yum install tethereal

Once you have identified the host, logon to the server and find server load. You may use w, uptimecommand to find server load. You may also use top and ps commands to determin Linux process that consumes most resource. To learn more about top command output, please read an article abouthigh volume traffic.

# uptime
 15:19:51 up 127 days, 5:39, 2 users, load average: 10.78, 8.68,
4.82

# top
top - 15:20:02 up 127 days, 5:39, 2 users,  load average: 10.78,
 8.68, 4.82
Tasks: 170 total,   6 running, 163 sleeping, 0 stopped, 1 zombie
Cpu(s): 3.5% us, 1.7% sy, 0.1% ni, 94.3% id, 0.4% wa, 0.0% hi,
 0.0% si
Mem:   2074924k total,  2046676k used,    28248k free,    58692k
 buffers
Swap:  4192956k total,      144k used,  4192812k free,  1553828k
 cached

  PID USER   PR NI  VIRT  RES SHR S %CPU %MEM   TIME+  COMMAND
14815 apache 25  0 52776  628 500 R 98.6  0.0  9:59.91 cw7.3

DoS and DDoS attacks occur due to running vulnerable software on your server(s). The attackers use known application vulnerability and security holes to compromise the servers in different network either by installing viruses and trojan horses (intrusion) or initiate DDoS attacks. To prevent DoS and DDoS attacks, you may take following actions.

1. Install Intrusion Detection System (IDS) such as Advanced Intrusion Detection Environment (AIDE). For installation procedure, consult Linux Gazzette. Perform regular system audits by installing and running RKHUNTER and CHROOTKIT to make sure installed Linux binaries are healthy. You may also install open-source network audit tools like NESSSUS, NMAP, and SAINT and perform regular network audits for vulnerabilities.

2. Implement Sysctl. Prevent ping attacks (ping of death, ping of flood, and smurf attacks) by disabling ping responses on the network machines. Enable IP Spoofing protection, and TCP SYN Cookie Protection. On Linux variant machines, follow sysctl configuration procedure.

3. Install advanced firewall and DDoS utilities. To secure your server and protect from DoS attacks, you may want to install APF, BFD, DDoS and Rootkit. To install those utilities, please follow DDoS Prevention: APF, BFD, DDoS and RootKit setup procedure.

APF: Advanced Policy Firewall
BFD: Brute Force Detection
DDoS: DDoS Deflate
Rootkit: Spy and Junkware detection and removal tool

4. Install Apache mod_evasive and mod_security modules to protect against HTTP DDoS attacks. For installation procedures, consult mod_evasive and mod_security how-tos.

For more technical information, please visit CERT or Wikipedia.

Flow Chart Troubleshooting of Video Card

General Printer Trouble

These steps should be taken when you're unable to get the printer to turn on and/or print properly.

Solution:

Printer does not have power indicator

First, make sure that the printer is on. When a printer is on it should have some type of light (usually green) that indicates that the printer is receiving power and is on.

If you do not have any indicator light make sure the printer is connected to a working power outlet by verifying each end of the power cable. Next, press the printer power button.

If after following the above steps your printer still cannot get a power status indicator it's likely you're encountering a serious printer issue and we suggest contacting theprinter manufacturer for additional steps and/or instructions on repair or replacement.

Cables not connected properly

Your printer should have two cables connected to it. The power cable and the data cable, the power cable should have already been verified as being connected if your printer has a power indicator light as mentioned above. Make sure the data cable (parallel cable or USB cable) is also connected from the printer to the computer.

Printer error (orange or blinking light)

After your printer has completed its initial startup you should have a solid green light. If the light indicator is blinking or orange often this is an indication of a printer error. For example, this could indicate a paper jam, issue with the ink or toner cartridge, or other serious error.

Because there is no standard to what a blinking light or orange light means if you're getting either of these we suggest referring to the printer documentation for troubleshooting steps or methods of determining what the status indicator is reporting.

No paper or paper jam

Without paper your printer will not be able to print. Make sure you have paper in the paper loaded into the printer paper cartridge or tray. Next, verify that no printer paper is jammed or partially fed into the printer. If you have one or more pieces of paper stuck in the printer these will need to be manually removed before the printer will print again.

Inkjet printer ink related issues

Often when you're encountering an ink related issue you're printer status indicator light (mentioned above) should be flashing. If this is not occurring you may want to simply skip to the next section. However, if you've recently inserted a new ink cartridge you may want to try the below suggestions.

See document CH000084 for additional troubleshooting steps that can be taken if your printer no longer works properly after replacing a printers ink cartridge.

Printer self tests

Most printers have a way of printing a test page. This page allows you to determine if the printer is physically working or not. This test is usually accomplished by holding down a series of keys. If you are not sure if your printer has this feature or how to perform it refer to your manual or visit your printer manufacturers web site.

In addition to testing the printer using the printer self-test Microsoft Windows users can also perform a software self-test to determine if their computer is able to see the printer and it's able to print. Follow the below steps to perform this test.

Microsoft Windows 98, 2000, ME, XP, 2003, and Vista users

1. Click Start, Settings, and open Control Panel.
   
2. Double-click the Printers or Printers and Fax icon.
   
3. Right-click on the Printer you wish to test and click Properties. If you do not see your printer listed your printer is not installed. See document CH000939 for additional information about installing a printer in Windows.
   
4. In the Printers Properties window click the Print Test Page button.
   
5. If the printer is able to print a test page, you're printer is installed and setup properly. However, if you're unable to print in other programs it's possible that the program you're attempting to print from has issues.
   

Older versions of Windows with older printers

If you are running an older printer and MS-DOS, Windows 3.x, Windows 95, Windows 98, or Windows NT, you can also attempt the below software test.

Get to a MS-DOS prompt - Additional information about getting an MS-DOS prompt can be found on document CHDOS.
Get to the root directory - Type cd\
Reroute dir to printer - Type dir > lpt1

The above should take the directory listing and print to the printer. If this does not print, refer to your operating system troubleshooting section. Extra Note: This will not paper feed, therefore press your FF or PP, or manually eject the paper.

Printer drivers

If your printer does not have any flashing lights and is connected properly it's possible you may be encountering a driver related issue. We suggest visiting our printer driver listing, which links to all major printer manufacturer driver pages and downloading the latest printer drivers for your printer.

Parallel (LPT) printers

If the printer you're connecting to the computer is an LPT (parallel port) printer we also suggest verifying the below suggestions if your printer is not working.

Parallel port in CMOS

1. Enter the computers CMOS setup. Additional information about entering the CMOS (BIOS) setup can be found on document CH000192.
   
2. Once in CMOS verify that your parallel port is enabled or installed.
   
3. Next, verify the printer or parallel port mode. This option will often have several different modes. If your parallel port is set to ECP mode, we suggest trying a different mode.
   

Other parallel device

If you have a parallel printer with other parallel devices such as a parallel scanner or zip drive, temporarily disconnect these devices to verify they are not causing your issue.

Additional information:

• See document CH000250 for additional information about installing and setting up a printer in Microsoft Windows.

Basic Network Troubleshooting

If a computer is unable to connect to a network or see other computers on a network, it may be necessary to troubleshoot the network. A network may not work because of any of the below reasons.

1. Network card not connected properly.
2. Bad network card drivers or software settings.
3. Firewall preventing computers from seeing each other.
4. Connection related issues.
5. Bad network hardware.

Solution:

Because of the large variety of network configurations, operating systems, setup, etc... not all of the below information may apply to your network or operating system. If your computer is connected to a company or large network, or you are not the administrator of the network, it is recommended that if you are unable to resolve your issues after following the below recommendations that you contact the network administrator or company representative.
Note: If you are being prompted for a Network password and do not know the password, Computer Hope is unable to assist users with obtaining a new or finding out the old password.

Verify connections / LEDs

Verify that the network cable is properly connected to the back of the computer. In addition, when checking the connection of the network cable, ensure that the LEDs on the network are properly illuminated. For example, a network card with a solidgreen LED or light usually indicates that the card is either connected or receiving a signal. Note: generally, when the green light is flashing, this is an indication of data being sent or received.

If, however, the card does not have any lights or has orange or red lights, it is possible that either the card is bad, the card is not connected properly, or that the card is not receiving a signal from the network.

If you are on a small or local network and have the capability of checking a hub or switch, verify that the cables are properly connected and that the hub or switch has power.

Adapter resources

Ensure that if this is a new network card being installed into the computer that the card's resources are properly set and/or are not conflicting with any hardware in the computer.

Users who are using Windows 95, 98, ME, 2000 or XP, verify that Device Manager has no conflicts or errors. Additional help and information about Device Manager and resources can be found on our Device Manager page.

Adapter functionality

Verify that the network card is capable of pinging or seeing itself by using the ping command. Windows / MS-DOS users ping the computer from a MS-DOS prompt. Unix / Linux variant users ping the computer from the shell.

To ping the card or the localhost, type either

ping 127.0.0.1

or

ping localhost

This should show a listing of replies from the network card. If you receive an error or if the transmission failed, it is likely that either the network card is not physically installed into the computer correctly, or that the card is bad.

Protocol

Verify that the correct protocols are installed on the computer. Most networks today will utilize TCP/IP, but may also utilize or require IPX/SPX and NetBEUI.

Additional information and help with installing and reinstalling a network protocol can be found on document CH000470.

When the TCP/IP protocol is installed, unless a DNS server or other computer assigns the IPX address, the user must specify an IP address as well as a Subnet Mask. To do this, follow the below instructions.

1. Click Start / Settings / Control Panel
   
2. Double-click the Network icon
   
3. Within the configuration tab double-click the TCP/IP protocol icon. Note: Do not click on the PPP or Dial-Up adapter, click on the network card adapter.
   
4. In the TCP/IP properties click the IP address tab
   
5. Select the option to specify an IP address
   
6. Enter the IP address and Subnet Mask address, an example of such an address could be:
   
   IP Address: 102.55.92.1
   Subnet Mask: 255.255.255.192
   
   
7. When specifying these values, the computers on the network must all have the same Subnet Mask and have a different IP Address. For example, when using the above values on one computer you would want to use an IP address of 102.55.92.2 on another computer and then specify the same Subnet Mask.
   

Firewall

If your computer network utilizes a firewall, ensure that all ports required are open. If possible, close the firewall software program or disconnect the computer from the firewall to ensure it is not causing the problem.

Additional time

In some cases it may take a computer some additional time to detect or see the network. If after booting the computer you are unable to see the network, give the computer 2-3 minutes to detect the network. Windows users may also want to try pressing the F5 (refresh) key when in Network Neighborhood to refresh the network connections and possibly detect the network.

Additional troubleshooting

If after following or verifying the above recommendations you are still unable to connect or see the network, attempt one or more of the below recommendations.

If you have installed or are using TCP/IP as your protocol you can attempt to ping another computer's IP address to verify if the computer is able to send and receive data. To do this, Windows or MS-DOS users must be at a prompt and Linux / Unix variant users must open or be at a shell.

Once at the prompt assuming, that the address of the computer you wish to attempt to ping is 102.55.92.2, you would type:

ping 102.55.92.2

If you receive a response back from this address (and it is a different computer), this demonstrates that the computer is communicating over the network. If you are still unable to connect or see the network, it is possible that other issues may be present.

Another method of determining network issues is to use the tracert command if you are a MS-DOS or Windows user or the traceroute command if you are a Linux / Unix variant user. To use this command you must be at the command prompt or shell.

Once at the prompt, assuming that the address is again 102.55.92.2, type:

tracert 102.55.92.2

or

traceroute 102.55.92.2

This should begin listing the hops between the computer and network devices. When the connection fails, determine which device is causing the issue by reviewing the traceroute listing.

How to Fix an Invalid Page Fault

Cause:

There are several reasons a computer may generate an Invalid Page fault error. Below are few of those possibilities.

1. Incompatibility or issue with the software programs.
2. Issues with the computer memory.

Solution:

Remove all TSRs

Disable or unload any TSRs or programs currently running before running the program causing the GPF.

Additional information on how to end task a TSR can be found on document CHTSR.

Delete all program temporary files

Delete all temporary files that may still be residing on the hard disk drive from currently or previously running programs.

Additional information on how to remove temporary file can found on document CH000225.

Run Scandisk / Defrag

Attempt to run scandisk / defrag on the hard disk drive as it could be possible your hard disk drive may have an issue causing the swap file or data files to become corrupt or invalid

Additional information on scandisk can be found on our scandisk page.

Additional information on defrag can be found on our defrag page.

Update software or check for software patches

If you are experiencing invalid page faults in only one program, verify that the software program is compatible with the operating system you are running the program within. Also verify with the manufacturer or vendor of the software program that there are no available patches or updates for the program that may help or resolve your issue.

Recently installed software / hardware

If you have recently installed new software or hardware, attempt to uninstall or reinstall that software / hardware to verify it is not causing your issue.

Bad Memory, invalid bits or physically bad memory

It is possible that bad memory can cause Illegal Operations. If you have recently added memory to the computer it is recommended that it first be removed to verify that you are not experiencing conflictions with the recently installed memory.

If no memory has been recently added to the computer it is recommended you follow all other steps found on this page before replacing the memory within the computer.

Common Error Beep Codes

AMI BIOS Beep Codes

• 1 Beep – Memory Refresh Failure (check RAM)
• 2 Beeps – Memory Parity Error in first 64KB block (check RAM)
• 3 Beeps – Memory Read/Write Error in first 64KB block (check RAM)
• 4 Beeps – Motherboard timer not functioning (may need to replace the motherboard)
• 5 Beeps – Processor Error (may need to replace the processor)
• 6 Beeps – Gate A20/keyboard controller failure (may need to replace the motherboard)
• 7 Beeps – Processor Exception Interrupt Error (may need to replace the processor)
• 8 Beeps – Display Memory Read/Write Failure (check video card)
• 9 Beeps – ROM checksum Error (replace BIOS chip or motherboard)
• 10 Beeps – CMOS shutdown Read/Write error (possible motherboard replacement)
• 11 Beeps – Bad Cache Memory – test failed (replace cache memory)

Phoenix BIOS beep codes 
They are series of beeps separated by a pause,
A 1-2-1-2 series would be like :
beep *pause beep beep *pause beep *pause beep beep

• 1-1-4-1 – Cache Error (level 2)
• 1-2-2-3 – BIOS ROM Checksum
• 1-3-1-1 – DRAM Refresh Test
• 1-3-1-3 – Keyboard controller test
• 1-3-4-1 – RAM Failure on address line xxxx (check memory)
• 1-3-4-3 – RAM Failure on data bits xxxx of low byte of memory bus
• 1-4-1-1 – RAM Failure on data bits xxxx of high byte of memory bus
• 2-1-2-3 – ROM copyright notice
• 2-2-3-1 – Test for unexpected interrupts

Award BIOS beep codes:

• one long beep and two short beeps – Video error (check video card)
• two short beeps – Non-Fatal Error (check RAM)

Troubleshooting Start up Problems

Why do startup problems happen?

Windows may fail to start for a variety of reasons, and generally speaking in order of decreasing likelihood here they are:

• Hardware failure
• Bad driver
• Corrupt file or volume
• System misconfiguration
• Virus infection

Let me elaborate. A common reason systems fail to start is because some element of the system's hardware has failed. This could range from the simple (someone kicked the power cord out of its socket) to the obvious (smoke emitting from the machine) to the mysterious (something transient that happens only when the moon is full or during sunspot minimum). Next most common is when you update the driver for some piece of hardware (or the BIOS for that matter) and the system won't boot afterwards. After that comes those mysterious messages we'll talk about shortly that usually indicate some key operating system file has somehow become corrupt or gone missing. Misconfiguration is another possible source of boot problems, but this is somewhat rare as in most cases you'll still be able to boot but one or more services may fail to start or your applications may not function as expected. Finally, virus infection can cause a system to fail to boot, but I've listed this in last place because I'm assuming you've got an antivirus solution in place and you're keeping the antivirus signature files updated, right?

Now that we know why Windows may fail to start properly, let's ask the logical next question: How can we know which of these underlying causes is the one that might be preventing Windows from successfully booting?

How to diagnose startup problems

Here is where we need to apply our brains and use a bit of common sense to determine what the cause of startup failure might be. Think of the previous list above as a list of disease-causing viruses, and now you have to play doctor and figure out which virus the patient (your sick computer) actually might have. For if you skip this step and try blasting the patient with every possible remedy in your doctor's bag, two things may happen:

• One of the remedies you try may actually make the patient worse and indeed could prove fatal.
• You'll waste a lot of time and the recovery of your patient will be delayed, and your boss may get upset with you as a result since her business is losing money due to downtime.

So careful diagnosis is a step you should always take time for and never avoid, and just like in the medical profession such diagnosis usually begins with your senses. For example, do you smell something burning? Better unplug your system immediately and wait for things to cool off, then open the case and inspect the damage. Do you hear your CPU fan making a slow grinding sound? Power down your system and replace the fan before your processor burns out and needs replacing. Is your video display flickering? Maybe try reseating the video card after checking if the video cable is seated properly.

OK let's assume its not such a simple and obvious problem. Instead, say you get a black screen with one of the following dreaded messages when you try and boot your system:

• "NTLDR is missing"
• "A disk read error occurred"
• "Invalid partition table"
• "Error loading operating system"
• "Could not read from selected boot disk"
• "Windows could not start because the following file is missing or corrupt"

Or you might get a blue screen (called a STOP screen) with some obscure message on it. Or if you're lucky you might make it all the way through the Windows splash screen to the logon box and then suddenly get a dialog box saying "One or more services failed to start". Or your mouse pointer might freeze and your system hang either before or immediately after logon. How can you match these symptoms to the underlying condition that might be causing them? First let's look at some possible "black screen" messages that can occur after the BIOS POST routine finishes but before the Windows splash screen appears:

Symptom	Probable Cause
"Invalid Partition Table" "Missing Operating System" "Invalid Partition Table"	Master boot record is corrupt due to hard disk errors or virus infection
"NTLDR is missing" "A disk read error occurred" System hangs after BIOS POST finishes	Boot sector is corrupt due to hard disk errors or virus infection
"Windows could not start because of a computer disk hardware configuration problem" "Could not read from selected boot disk" "Check boot path and disk hardware"	Boot.ini file is corrupt, missing, or needs updating.
"Windows could not start because the following file is missing or corrupt"	Boot volume is corrupt or the referenced system file is missing.

In addition to these error messages, a variety of other startup problems can occur including:

• Blue screens. These are typically caused by hardware failure or driver problems but can also be due to virus infection.
• Hung system. These are typically caused by buggy drivers or by registry corruption but can also be due to virus infection.
• Dialog box saying "One or more services failed to start". This is typically caused by misconfiguration or registry corruption but can also be caused by application incompatibility of some form.

So what should you do to resolve such problems?

How to resolve startup problems

Like a doctor's mysterious black medical bag full of medical instruments (at least in old movies on TV) the system administrator also has a set of tools provided by Microsoft for resolving startup problems like the ones in the table above. In a nutshell, here's a quick inventory of the main tools:

• Last known good. Restores the HKLM\System\CurrentControlSet portion of the registry its version during the last successful logon to the system.
• Safe mode. Starts Windows with a minimal set of drivers and creates a record of which drivers load in %windir%\Ntbtlog.txt.
• System Restore. Windows XP only feature to restore system to previously saved configuration.
• Recovery Console. Boots to a command line that allows you to run various commands, see this article by Johannes Helmig for more info.
• Automated System Recovery (ASR). Restores the boot volume from backup, see this article by Johannes Helmig for more info.
• Repair. Run Windows Setup from your product CD and select the option to try and repair your installation.

Which tool should you use to address each of the symptoms we described earlier? Assuming there is no obvious hardware problem (no funny smell) and you've already asked yourself the Golden Question ("What was the last thing I did to this system?") then here's a quick outline that maps the type of knife (may be several in order of severity) to the kind of surgery (underlying problem or visible symptom) you need to perform on your system:

Problem/Symptom	Tool(s) to Use
Corrupt master boot record	Recovery Console (fixmbr)
Corrupt boot sector	Recovery Console (fixboot)
Corrupt or missing boot.ini	Recovery Console (bootcfg /rebuild)
Corrupt system file	Recovery Console (chkdsk) Restore from ASR backup Perform a Repair install
Corrupt registry	Recovery Console (chkdsk) System Restore (XP only) Restore system state from backup Perform a Repair install
Blue screen	See this resource first Last known good System Restore (XP only) Safe mode (roll back suspect driver using Device Manager)
Hung system	Last known good System Restore (XP only) Safe mode (roll back suspect driver using Device Manager)
"One or more services failed to start"	Don't logon! Reboot and select last known good, log on, undo the last configuration steps you performed. Safe mode (undo last configuration steps you performed)