Newly built computer wont boot up

HELP, please
I have built several computers before but this one has stressed me to no end.
This is an Intel Core2 Duo 3.06 GHz (other stats below) that starts, shuts off, and starts and shuts off.....HELP!
I've taken it apart to insure that the cpu is seated properly...not too much thermal paste, heatsink/fan sits perfectly and is snug. Changed and checked psu. What have I done wrong????

- GIGABYTE GA-EP43-UD3L motherboard
- Intel Core2 Duo E7600 Wolfdale 3.06 GHz, FSB 1066MHz, L2 Cache 3MB, Core 45nm
- 8 GB G. Skill - DDR2 - 1066, PC 8500, 2GB x 4
- SPARKLE GeForce 9400 GT 512MB 128-bit DDR2 PCI Express 2.0 x16 HDCP Ready Low Profile Video Card
- Maxtor DiamondMax 100 gb 7200 rpm
- WINDOWS XP SP3