Computer Building Issues

Ok, basically i just bought brand new compute rparts on newegg.com. I put it together and everyhting but now when i go to turn it on it stays on for a second then shuts right back off. The only thing i can think of is that while putting it together, one little piece wouldnt fit. The Computer Led, which according to my manual should be a 3 port plug, was only a 2 port plug, so i left it unplugged, which i thought should be fine since its just the power is on light.

Please, any and all suggestions will been taken thankfully.

When I built my PVR it was doing the exact same thing, it turned out to be the power switch.

To test it, unplug the power switch from the mobo, take a screwdriver or paper clip and jump the power jumper. If it stays on, then you know it's the power switch.

If not, then probably something is overheating.

ok, i have deemd the power supply as working using othere various sites to learn how to jump the power supply. regardless, the computer still doesnt work. Any othe suggestions?

If it still will not boot after trying what Makari suggested, your motherboard could be shorting out. To find out if this is the problem you need to set you motherboard outside of the case to see if it runs.

I had this problem once, my computer would start for a few seconds then shut right back down. I took the motherboard out and set it on a box and it ran all night.