I would say that you are either over-heating or you have ram troubles.
What computer do you have?
What version of windows?
To check the temp you can try Motherboard Monitor 5. It will show you your temps.
http://mbm.livewiredev.com/
To check your RAM use Memtest86.
http://www.memtest86.com/
Also if you want to try and get a better idea of the error (XP), Right click my computer -> Properties -> Advanced -> Settings under Startup and Recovery -> Uncheck Automaticly restart
Jkrohn