I just finished building my second computer yesterday and after checking to make sure everything looked right I pressed the power switch to start it. The system started to turn on but immediately blinked off after about a quarter-second. After unplugging everything I checked to see if all the components were secure and nothing extraneous was contacting the mobo, then I attempted to start it again with the exact same results. After searching this site for possible solutions, I found a list of things to check and went through them one by one. With no solution found, I disassembled the components, pulled the mobo out and reassembled it outside the case. When I completed the power up circuit the system started perfectly. I confirmed that the case standoffs are in the right positions and only have as many as are mounting holes in the mobo. I reinstalled everything and attempted to start with the exact same results as before, no powerup. Anyone have any ideas on what I may have missed?
Definitely sounds like the motherboard is experiencing a short circuit and shutting itself down to protect itself.
I think you should:
1) Take the motherboard back out of the case
2) Take all the stuff out of the motherboard except RAM and CPU
3) Take as many standoffs out as you think can be taken out without having the motherboard touch the case. You should be able to get by with about 3 if you choose them strategically.
4) Plug the PSU back in and try to start it.