Possibly the mobo, but try a PSU tester (http://www.newegg.com/Product/Product.aspx?Item=N82E16899705003)
or another verified working PSU to swap out. Start going through it one wire at a time so to speak. Pull ALL peripherals like CD/DVD Rom, hard drives, expansion cards, etc. You want to take this system back to bare bones. Just the psu and cpu, use on-board vid if its there so you can take the vid card out too. Let us know what you find from there.