I agree with Peter. Some hardware thingie in your PC is bad, indeed, most likely the RAM.

Windows 2000 automatically reboots on occasions like that. Very frustrating, I know.
You can actually disable that "booting", so you can see the actual error.

Go to system properties, (right click my computer, then properties) -> advanced -> reboot and repair eh stuff (option on the bottom) -> uncheck third option with system errors ("automatically reboot")

I'm trying to translate the exact steps here since I'm using a Dutch windows 2000, but you should be able to find this no problem.

Of course, then the painful process of debugging you hardware only begins...