Newly built computer powers on, then powers off

[beto5243]

I've recently finished putting together a new computer (I'll put parts at the end), but when I turn it on everything seems to power on for half a second (all fans start blowing, some LEDs turn on) and then it turns off. What are some possible causes for this, and how would I identify/fix them? Thanks in advance.

Specs:

GIGABYTE GA-P67A-D3-B3 LGA 1155 Intel P67 SATA 6Gb/s USB 3.0 ATX Intel Motherboard
Intel Core i5-2500K Sandy Bridge 3.3GHz
MSI R6970 Lightning Radeon HD 6970 2GB 256-bit GDDR5
G.SKILL Ripjaws X Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600
CORSAIR Enthusiast Series TX850 V2 850W
COOLER MASTER V6 GT RR-V6GT-22PK-R1 120mm

If you need anything else, let me know. Thanks.

[JPMiller]

reseat everything, make sure the heatsink is secure especially.
then just unplug everything but the memory, video card, CPU/Heatsink and power supply.