Do you have the 4 pin cpu power plugged in as well as the 20/24 pin.
One other thing I can think to try is to take out your mobo from the case and make sure its not shorting in the case.
Assemble it on a phonebook or some thing and see what happens.
Also when you try this. Take everything off the mobo including the cpu. After you got a bare mobo reset the cmos on only the mobo nothing else pluggged in. Assemble on phonebook and see if it powers up.
Did you use the brass standoffs to mount the mobo, and the paper washers?