My friend showed me his dead computer. When I powered up, the green LED on the motherboard didn't even come on. He had bought a new $40 500W power supply and he wanted me to install it. I did, and now the green mobo LED did come on, but nothing else powered up, not even briefly. (I guess I mean the CPU fan not starting, and no beeps. What other signs would there be?) I unplugged all drives and removed all PCI cards. The power supply is plugged into the mobo in 2 places, the 20-pin connector and the 2x2 connector. Still dead.

Then I noticed that I got a small electric shock when my left hand touched a metal sink (ground) and my right hand touched the computer case. I measured the voltage difference to be about 75V! I unplugged the Power Supply from the mobo with no change. I removed the Power Supply from the computer completely, so the only connection was the power cable from the wall to the Power Supply. The case of the Power Supply still measured 75V! When the Power Supply was switched off, still 10V.

I decided to try the old Power Supply, plugged into only the power cable, and the same voltage measured on the case! Why is this happening? Is this normal?

I did drop the new Power Supply once. I don't know what may have happened to the old Power Supply.

Would this high-voltage case harm the motherboard? Maybe it wouldn't, because the mobo is electrically isolated from the case.

At this point, I'm not sure if the motherboard is dead, but first I want to clear up this high-voltage question.

Did the motherboard kill the new Power Supply, and that's why it's shorting?

Both power supplies pass the paperclip test (ie their fan starts when I connect its green wire to its black wire).

The motherboard is P4C800, and the system had (before I unplugged everything today) 4 hard drives, 2 DVD drives, 3 case fans, a video card and a network card.