Whenever I am in a situation like you the OP (and it does happen a lot, same symptoms).
1- Check with a new/other PSU + check every connection also from the PSU to the Motherboard, make sure it's not some power connector that isn't properly connected (Yes it happens, sometimes we forget about the basics
2- CPU check, while a dead CPU wouldn't shutdown the system at start, it would just give most of the times no bleeps (no POST) at start and a black screen, remove it and try to start the system without it! (Yes it won't properly start but if the system stays say 10s on you now know the CPU might also be dead or also causing problems)
3- At this point if you're looking just for the system to at least be on, remove (the connections to the board) of everything, disks, DVD's, any usb connectors, Firewire, etc.. Just have power to the board, and the the power and led's buttons of the case!
4- If it still doesn't work, sure make a reset of CMOS, etc.. But if the problem still exists you have a dead board, don't spend money on parts to try, just replace the board!
PS: Oh and yes the CPU fan spins for a second and stop's, yep I've seen before