Dell XPS 200 power issue

I have a Dell XPS 200 that died. I was told it was the motherboard so I replaced. No dice. Discovered issue with PSU. Replaced. Got power to the machine but still wouldn't turn on. Had a flashing yellow light on power button. I disconnected all devices and checked power each time I attached a device. I ended up discovering that only when I added the CPU chip did it fail. So I replaced CPU (Pentium D 845). I still have the same issue. I've tried both motherboards with same result. When I put in the CPU the power fails. Without it the machine powers on. What could it be?