Help with A7N8X please

Here's the problem. When I hit the power switch, it starts up, but it shuts itself of after a few seconds. No POST beep codes. All the fans stop running, but the green status light stays on. What could be the problem?
Here's what I've done so far. I took the mobo out of the box, placed it on a peice of foam, installed the CPU(XP 2200)/Heatsink, installed on stick of 256MB PC2100 DDR, placed the video card (Radeon 9700 Pro), pluged in the HD to the primary IDE connector, plugged in the keyboard and monitor. I set the Serial IDE jumper to disable serial ATA, verified that the FSB jumper was set to 333/266 and reset the CMOS. I've reseated everything one at a time to try to pinpoint any troublesome components. I'm stuck. What do you all think? Any help would be appreciated.

1st Ranger

Clear the CMOS. Unplug all devices, except VGA, and I mean ALL including hard drive/s. Try to get it to POST. If sucessful, go from there by adding one device at a time.
Scott

Ditto - unplug everything except for the basics. You only need cpu, ram, video, keyboard and mouse. Make sure you're clearing the cmos by removing power from the board, moving the jumper to clear, and then hit the power button. Replace jumper to normal position. Often trying to clear cmos with power applied doesn't work.