OK, I'm fixing a computer for someone I know. He just dropped it off and said it wouldn't turn on at all. I diagnosed it and decided it was the power supply and ordered a new one. Now here's my problem, one I haven't encountered before. With my spare PSU hooked up to the computer it boots up just fine. When the new PSU is hooked up to the computer the CPU fans spins up then stops, the power light flashes once, and the PSU fan just jerks once. But when I take the new PSU and hook it up to my HTPC it boots up with no issues. Same power cord. WTF am I missing?