Computer Rebooting

Hey folks,

Got a strange problem here with my second pc.

A few months ago it all of the sudden started rebooting out of no where. It
rebooted while in Windows and also during bootup, so I thought it was a
windows problem, but it's not. It even reboots itself while I'm browsing
the BIOS or in Linux, or even 2 seconds after I turn it on (before any OS
starts loading).

So, I thought it was the power supply. I bought a brand new 450 watt one,
but it still rebooted. (I've since then moved that power supply to another
pc and the power supply works without a hiccup).

So, I disconnected all peripherals, removed every PCI card, and put in new
memory. But still it constantly reboots. I then even disconnected the
cdrom, every hard drive, and the floppy drive....but it still reboots.

I also tried plugging it direcly into the wall (bypassing the surge), and
tried different power outlets in the house, but it always rebooted.

So, I think the problem is either the motherboard or CPU. Do you folks
agree (well what else could it be)? And if so, which one do you think it
is? Is there a way for me test which one it is? I'm leaning towards the
motherboard, but I don't know how a CPU behaves when it fails.

Thanks for the help!