Hello,
Make sure that the motherboard is not touching the case or any other bare metal in the case. The symptoms you dexcribe sound like this could be an issue. If possible try and get the motherboard up and running before you put it in the case. Place it on a big book (like the yellow pages) and make sure that you don't put any computer bits on the carpet.
If it still doesnt power up you can rule out shorting on the case.
HTH
Kev
