Question : Problem: PC keeps re-booting, not a virus

I have a Windows XP PC that reboots several times a day.  I've thoroughly checked it for viruses, that's not the the problem. It just powers itself off, then comes back up.  What can be causing this?

Answer : Problem: PC keeps re-booting, not a virus

Is it powering off or simply rebooting?

Possible causes:

power supply
memory
bad voltage regulation circuit on motherboard - check capacitors for signs of electrolyte leakage or deformation.
faulty driver
Windows glitch