I have a dell dimension 8400 which has developed a problem. The problem is when I try to turn it on, the power button light turns solid orange and nothing else happens. I can't hear any HD or fans running and none of the diagnostic lights on the back panel are on. Just the orange light on the paower button is on. I have read this may be a processor or motherboard failure, but I want to know how I can test which one it is to avoid throwing expensive parts at the problem.