The Mystery of the rebooting PC

I have a white box system with a P3 600mHz chip on board and 256mb ram and a 10gig harddrive running win2k pro. I am having a problem because every time I go to boot it up it gets halfway through the boot process and just restarts (right after the Win 2k splash screen goes away it restarts). Can anyone help me with this problem? I tried changing the memory and even taking the battery off of the mainboard for 1 hour so the bios can reset and that didn’t even help.