No POST

A customer’s machine displays not even the slightest signs of a POST when I boot it up. the machine does recieve power (fans and drives are spinning up), but all else remains dead. I have checked the RAM chip (128MB PC100), the processor (PIII 800MHz), and the video card (ATi Rage Pro AGP). Each of these pieces have been replaced and the system still remains dead. This system had worked periodically even earlier today, so I’m not too quick to charge the motherboard as the culprit. That is my last resort. I also moved the RAM and tried a PCI video card…nothing!

HELP!!