Sounds smilar to what was happening with my brothers PC after he short circuited it by trying to stick the wrong end of a usb cable into a USB slot.

Ended up being the Mobo and CPU.

I'd try and fnd the thread but the search doesn't seem to be working too well ??

It was called Spark Causes System Shutdown or soemthing to that effect, if I can find it I'll post a link.