Wow, you've really opened a hornets nest here..... reason why noone has responded really, is because you need to be more descriptive... Most likely your system is blue screening, and rebooting automatically, as WinXP is setup to do. You can turn this option off, but I'm not at home to explain exactly how to you... In turning it off, you can get the blue screen error message and get an Idea where the error is coming from...
In addition... when is it rebooting? Games, from the desktop?
What kind of hardware do you have?
Spook