do this take out the video card and all ram but one stick and see if the mb power on. if not see if the cpu/video /ram led is on. if it not a short it may be that the cpu not seated right or there a bent or missing pin. i would pull the cpu..check for bent pins under the cpu and then check that the cpu seated. check the back of the mb make sure there not shorts to the mb case or cpu cooler. myself it sounds like you have dead leg in your power supply.i would see first if you have another ps for testing or get that one load tested at micro center or or geek squad.