Hi all, I have put together a new system. Motherboard has a P4 connection and the PSU is 500 watts. All was well at first, PC would POST and I was able to install Vista. Switched PC back on and it would not POST (no beeps)and no video, all fans spin, HDD spins and DVD will open. Tried another PSU (300 watts) from another PC and everything was fine. I then tried the new PSU in the donor PC (which does not have a P4 connection) and it booted windows. Checked voltages at the P4 connector and had 12 volts. Retried new PSU and it still would not POST.
Have not got another motherboard with a P4 connection to try, but does this mean the PSU is faulty. Want to make sure before returning for replacement/refund.

Answers on a postcard please?