Gigabyte M55-SLI 4
AMD 64 x2 4200+ 2.2 GHz
e-GeForce 8800 GTS
40 GB Seagate IDE HDD - master
120 GB Seagate IDE HDD - slave
2 GB (2x1GB) A-DATA DDR2 800
Zumax X3 series ZU-550w

This system worked for about 1 week and today it does as the title says. I push the power button on the front panel of the case and the LED lights for the HDD flash on then off one time, the same for the PSU fan in the back. In order to get the lights to flash again I have to depress the power button in front for 10 seconds or remove the power cable, turn the PSU off, plug the power cable back in and then turn the PSU on. This lead me to believe that something is making the PSU shut itself off, but I can't figure out what. There's nothing that could cause a short that I can see, and there wasn't any kind of power surge earlier.

Also, my mouse is getting power, and the ethernet LED in the back flashes once when I turn the PSU on with the power cable in.

Any kind of help would be appreciated.