Also this will be a pain in the ass, but take everything out of the mobo apart from the CPU and 1 Stick of ram and try and get the post to work...if it does, add a hard drive, and repeat, and keep going till it fails, when it fails, theres the problem.
Also reset the CMOS...pull the battery for 5 mins (this forces a hard reset of the settings) put it back in and fire it up, everything that may have changed in the bios will now have reset to default.