Athlon 2000+
A7V-333-X
512Mb Crucial DDR 333
120Gb Maxtor HD
Creative GeForce2
Creative Live! Sound
Enermax 300W PSU
Volcano 9+ HSF

I have built the above computer, but it won't boot up, or POST. There are no system beeps. The motherboard LED lights up. When I press the power button the fans spin for about a second, then lose power again.
I have stripped the machine right down to the motherboard, graphics card, one memory module, CPU/HSF. Still the problem remains. I installed a new motherboard thinking I had damaged the old one, same problem. Same with the processor. I have tried using the computer in different rooms thinking the power outlet was faulty, also using different power leads, and a different power supply unit. I have tried to use the machine with / without a Thermaltake shim with no success. Memory, motherboard and processor appear physically undamaged. I have also tried powering up the system through the reset button, thinking the original power button was damaged. Still no luck.

Any ideas?

In this case the only way out is to check CPU, video and RAM (and probably PSU) on other (known working) motherboard. If those are working then check working CPU and other components with your MB and you will find what the problem is.