So here's a quandary. I found this old PC that won't shut off completely. I press the power button and the PC turns off and immediately turns on again.

This happens with no storage or OS loaded at all, so that rules out it being a power supply thing. Hell, just for giggles, I booted to a live CD, unplugged the motherboard front panel headed and shut down the PC through the OS and it still happens.

Any ideas what it could be?