You're likely getting a blue screen of death due to an error in some program that is running (error possibly triggered by a hardware issue, like ram or psu). Right click my computer, go to properties, and the advanced tab. I don't have a win2k/xp system with me right now, but I'll guess it's under something like "shutdown and recovery". There'll be a checkbox for "automatically reboot" and a number of memory dump options.
Assuming this is the problem, next time the compute would reboot, you'll be left with a blue screen of death. If it ends up being a program or drive issue instead of hardware, often times the BSOD will leave some droplet of information that could point you to the right one. ----------------------------------------------------------------------------------
...but I'm just a C man trying to see the light