PC rebooting on its own.

The PC that I am working on Keeps rebooting on its own. I flashed the bios and also deleted and recreated ntfs partition and reinstalled windows 2000 pro. It was fine for a day but then the user called and said it was still happening. Which item would it most likely be. The power Supply, Memory,or the mother board. It Does not hang or anything else like that. It just Reboots.
Robert Hodges