Well, if there was a short, it would have to be from the motherboard to the case. That's the only place it really could be, unless by some strange happenstance one of your drives went spectacularly toasty. You'd prolly notice that, though. Black marks and such. Maybe try removing the motherboard and making sure there isn't something bridging a ground where it shouldn't.
Wait... I know where I've seen this problem before...
Check and make sure the voltage selector switch on the PSU is set to 120V instead of 240. Both on the old one and the new one.