The symptoms my previous pc showed before the motherboard died was:
It started shutting down spontaneously. When I power up the pc, it'll post. But as soon as Windows or Linux tried to load, the power shut down immediately. I tried the PSU on a different pc and it worked fine.
I tried booting from a Windows install cd or a Linux Live CD. The same thing. Immediate shutdown. It wasn't the PSU. It wasn't the ram. It definitely wasn't the video card.
Upon close examination of the motherboard, I saw something similar to this:
http://en.wikipedia.org/wiki/File:Bu...yeh_caps_2.jpg
Bulging and/or leaking capacitors.
As it got worse, the power button wouldn't even work after shutdown. The activity light simply blinked. Pulling the plug and putting it back in allowed the pc to power up again. But, it just shutdown again as soon as an OS tried to load.
If you have similar capacitors on your motherboard especially behind the cpu, kiss your motherboard goodbye unless you can get it fixed for cheap there.
Here, it's not worth it. It's cheaper to buy a replacement motherboard, one with solid state capacitors.