Do you have the ATX 12v four-pin connector connected to your motherboard?
It will be near the mouse and keyboard sockets,
the connector would come from your PSU and have black and yellow wires.
Are you sure that the motherboard is not shorting to the case?
This could be a loose screw or the wrong stand-offs being used.
Please be careful if you try this
You could try taking the motherboard out of the case and put it on a piece of cardboard.
Put in your video card and ram then connect the PSU and see if it will boot.

