XP can be set to reboot automatically without presenting a blue screen on a system error. To deactivate the automatic reboot, right click My Computer, select Properties, select Advanced tab, and in Startup and Recovery hit the Settings button. Under System Failure, uncheck Automatically Restart. (Whoo!) Now at least you will get an infamous blue screen if it is a system error causing your trouble, with a pretty much indecipherable error message which may (but probably won't) help you narrow down the source of your grief.