I'm gonna have to lean towards what waffle is sayin for now. You said you're missing one of the screws in the mobo. I know you said that you did that on the older board as well and it still worked, but that might not always be the case. My first rig did the same sort of thing (shorting out and wouldn't start) cause i forgot one of the standoffs.
Get that screwed in. If it don't work, start goign piece by piece. Try booting with just the cpu in, listen for beep codes if it starts, then put the ram in and try, listen for beeps, then put vid card in and try etc.
3 fans shouldn't make any difference at all when it comes to power requirements.