Motherboard or PSU issue? HELP!

Hello,
When I press the power button, an led on a fan just flickers.

Build:
CPU: Intel C2D e4400
Mobo: AsRock ConRoe1333-1394
PSU: Topower ZU500W
GPU: MSI HD6450
Case: Xion XON-180

A few parts I had on an old Pentium 4 computer and they worked perfectly.
These are all the new parts.

Also, I've found that if I unplug the ATX 12v 4-pin connector, the PSU starts up, and provides power to the motherboard only, not the CPU. I test the PSU on another computer. It works fine. I actually found that it has a slight generator hum when it's plugged in. But, when I plug it into the 20-pin mobo connector and 4-pin ATX, the hum stops, and it won't work. I swear it's the motherboard... but I don't want to spend more money....