New build will only power on for a second!

sounds like you shorted something to the mb. mb and ps have a dead short feature to keep them from turning on. i would first start by looking at your cables. are both atx power connector plugged in. on new my there are two a 24 pin and a 4/8 pin one near the cpu. if the small one not plugged it you get that power on twich then nothing. for the mb did you use the 9 borwn standoffs to hold the mb off of the mb tray?? a lot of people make the mistake and screw the mb to the tray and short it out. check the i/o shield..the long fingers tend to be too long and can short the mb out. check if you used an after market heat sink that it not grounding out the mb on the back side. i would also put the mb ontop of the shipping box and see if it power up. if it does then you had a short. if it wont it could be a bad power supply or a bent pin or ram not installed right. the hard part is looking under the cpu and on the cpu for a bent pin or thermal paste that gloobed over the side of the cpu and is on the pins.