i have just finished replacing one of these mobo's, the last one lasted six weeks. completely died, got replaced and have put back in the case tonight. on first boot, exactly the same thing happened. power on, fan spins then off again, one second later, power on again goes right into windows.
don't know if its an Asus thing (as its the first time i have used an asus board) probably be the last as well.
i have just shut down the pc, will switch on in a mo and see what happens.
no problem this time, might just be after mains power has been off.
will try that and report back.