no video

I am trying to troubleshoot a computer for my customer. When I turn on the computer, I do not get any video or any beeps. The only thing that seems to work is the cpu fan. I have tried changing the motherboard and processor. I have also tried changing the video card, using both AGP and PCI. I have also taken out every component not needed to at least POST. It is an AMD Thunderbird 1200 cpu and an ECS K7S5A Pro motherboard. I need this answer right away!