I just built a new pc, and though I've built many computers in the past six months, I have never come across this problem. It has never posted, and it constantly reboots after hitting the power button on (it runs for a few seconds then shuts off automatically.) The CPU fan will not work when plugged into the 4-pin cpu fan header on the motherboard, but works fine when correctly attached to a 3-pin fan header, however neither affect stability. I can get the system to come on and stay on when the ATX 12v plug (the 4 pin plug to the top left of the LGA 775 socket) is not connected, but for obvious reasons the video card will not power on, thus making it pointless. I've tried swapping ram slots (I have no other DDR3 laying around to try), reseating the cpu, cpu cooler, video card, and have tried multiple power supplies. Here are the exact specs:

MOBO: GIGABYTE GA-EP45T-UD3LR 775 RTL N82E16813128371
CPU: INTEL|C2D E8400 3G 775 45N R N82E16819115037
PSU: OCZ|OCZ700MXSP 700W RT N82E16817341018
RAM: 2Gx2|OCZ OCZ3RPR13334GK R N82E16820227293

If anybody has any idea what is causing this, please let me know, Gigabyte's tech support has been slow to even read my inquiry.