2 PCs won't power up, please help!

Hello everyone,

I recently built a PC for my sister. Components are as follows:

CPU: Intel C2Q Q6600
Mobo: Gigabyte GA-p31-s3g
GPU: Geforce 8800GTS 320 mb
PSU: Antec Earthwatts 500W

Ok, so when I first assembled everything, the PC turned on, the fans spun, but I forgot to plug in the PCI-E plug into the video card Video card - Wikipedia, the free encyclopedia so I got the loud screeching noise. I turned off the pc, unplugged everything and plugged in the PCI-E plug. Turned the PC back on, and I got no video on the monitor. So I assumed that I forgot to plug something in, something was short circuiting the mobo, etc, so I took it apart to examine the computer. When I got everything in order, the PC would not turn on at all -- no fans would spin, no lights, nothing.

I posted on some boards and figured it was my PSU. So I took the PSU out of my old computer that I built a year ago (an Antec Liberty 400W PSU) and tried it with my sister's computer. Nothing happened again. Now I tried to put the PSU back into my PC, and now the same problem happens with my PC. No power, no fans, nothing. When I hit the power switch, I hear a little click (only sometimes), I think from the power supply, but then nothing happens. That only happens when I remove and plug in the power cord from the PSU and press the power button.

Basically, now I have 2 expensive bricks laying around in my house and need help! I have no idea what to do, and am freaking out because I didn't have to use my PSU on her computer, and now I have 2 unusable PCs instead of one.