This is problem my son is having. Every time he logs on the the internet the computer shuts down and restarts itself. Sometimes it lasts 30 seconds or it can be up to 5 mins.
He's running xp pro and internet explorer version 6. He's just told me he doesn't bother downloading updates for windows so i don't know if maybe its a bug there's a patch for but he's going to have trouble downloading it now anyway.
Right-click on My Computer, click Properties, click the Advanced tab. Under Startup & Recovery click Settings. Under System Failure unticck the box in front of Automatically restart. The next time it should not restart but display an error code and message on a blue screen. That would help to pinpoint the problem.