PC turns on and off repeatedly.

Hi, i just finished my first new build with the following components;

gigabyte p55a ud3, i5 760, gskill ripjaws 2gb pc3-12800, antec earthwatts 650w and EVGA geforce gts 450.

Every time i try to boot it turns on for 2-3 seconds then turns off, then about 10 seconds after that it turns on and off again. This cycle continues until i turn the psu off. I've tried booting it with only one stick of ram and the graphics card attached to the motherboard to the same end and ive swapped about the ram as well.

Any suggestions would be greatly appreciated. thanks.