You need to turn automatic reboot off so you can see what's on the BSOD.
Right click "My Computer" and select Properties (or go to Control Panel->System), go to advanced, click Startup & Recovery and make sure that the option marked "Automatically Reboot" is NOT ticked.
When your PC crashes next time, it will stop at the BSOD. We'll need the information that's on that screen before we can help.
You may also want to check your event logs to see whether there are any strange messages in the system log.
 
Rgds
AndyF

bold text
Hello Friend
I have also send a problem about XP Restart .
my xp restart when i connect on internet . i thik a virus on net woh restart my pc.if u have information about this problem so i am very thankfull to you .
ok

Do the steps above and let us know what error is displayed on the blue screen when your PC reboots.
Once you've done that, maybe we can be more helpful
 
Rgds
AndyF

Sounds like the sasser virus. Use this link below to grab the removal tool off symantecs website.
 
http://securityresponse.symantec.com/avcenter/venc/data/w32.sasser.removal.tool.html

Also, check out http://www.ntcompatible.com/story29322.html