a7n8x Won't Post or Beep

Hello,

I have an A7N8X deluxe motherboard with a 2700 Athlon XP. After installing a new heat sink and swapping memory around my computer no longer boots. When switched on it powers on and the fans spin, DVD drive lights up and HD starts to spin. Then after about 2 seconds it switches itself off. There are no beeps or stuff on the monitor. After swapping memory modules, removing all unnessacary hardware and trying another (but dodgy) video card there is no change. I guess it is the CPU or motherboard, or maybe the power supply.
How can I tell what is causing the problem?

Thanks for your help.

 

Further Info

I have tried to work backwards by removing or replacing all that I can.
If I use a different power supply then I have the same problem.
If I remove the CPU and everything else then I get a POST message that no CPU is installed and the mobo does not turn it self off.
If I put the CPU back in then it turns itself off and the POST message is "System fault ...." but does not get a chance to finish. So it looks like the CPU, however I have had a faulty CPU message before on an ASUS mobo and it turned out to be the motherboard.
I guess I can only truly know by buying a replacement CPU