Some motherboards have have a square ATX plug that people sometimes forget to plug in. If you have this make sure it is plugged in.
If that isn't the problem then you can try doing a bench build to eliminate the board shorting out against the case. To do this remove the board from the case and lay on non conductive surface like cardboard or the motherboard pad. Install processor, heatsink/fan, one stick of memory, video card. Hook up powersupply, monitor and the start button from your case. Start it up and see if it will post. If it post then check to make sure all of the mounting post are in the correct spot and you don't have extra. Also look for other places it may be shorting out.
Folding for Team #34106