I once had it happen to me that the reset switch on the case was stuck. So the motherboard worked fine OUT of the case, but every time I put it back into the case the thing wouldn't post. (because each time I would connect those tiny led/speaker/reset switch connectors again without knowing that that was the problem).

The moment I pulled those wires while the PC was on the thing posted no problem!

So I agree with Andy : assemble the PC OUTSIDE the case and connect nothing to the motherboard but keyboard, mouse, videocard and power.
If it doesn't work then you can pretty safely say that your motherboard is kaput.