I just got done fixing a computer with the same problem. I had to swap out the motherboard. Stripe the system down to the CPU, Ram, and Video Card and see if it boots. Next If you have a spare Power Supply try hooking that up to your striped down system and see if that boots it. If not it may be the motherboard.
Has this system ever booted up before?
Did you place the motherboard on standoffs?
Is the CPU fan plugged in?