It could be a couple of things...
1. Make sure your cpu fan power cable is plugged in correctly (not into a chasis fan plug-I've done this a few times before
2. Reinstall your cpu, make sure it has thermal compound applied properly and isn't overheating
3. Double check that there isn't a short between the mobo and the stand off screws
4. I've also seen some compatibility issues with Asus mobos and newer chips that require a new BIOS to be installed. That id the trick on my current pc and my dad's who uses the same mobo (both Asus' of course). Give that a shot too just be careful if you've never flashed a BIOS before and do exactly what the instructions say.