You can get Windows XP to give you a BSOD, rather than restarting, then use that info to track down the problem.
I also remember theres a dump file somewhere that will detail each crash, in the admin / performance tab
If I were a betting man, I'd say if you didn't change your power supply, it could be the source... but it also could be a heat or hardware issue of some kind.
If you're not exactly sure of your systems innards, try this:
Belarc advisor It'll show you everything and more.