I have a dell system -- Dimension XPS D300.

Few days ago my computer failed to boot-up. When I switched on the power, the was no power supply. I suspect the power supply failure. So, I went to a shop and bought a new power supply.

I install the new power supply and switch it on. There is still no power. I tested the power supply in the shop and it is working fine.

So, I followed the Reference and Troubleshooting Guide and unplug all the connected devices from the system board. And also all the power connections to those devices. So, now the power supply only connected to the system board. The system board has nothing on it (no CPU, no RAM, no Cards, all cables are removed) except the cabal that connect the Control Panel Connector (J8F2) to the power button. The 3.3-V power input connector (3.3) is also not connected. Then I switch on the power supply. There is still no power. The fan is still not spinning. No matter how many times I pressed the power button, there is still no power.

I don't know what is damage. From my observation, there are 3 possibilities:
1. The system board has damaged.
2. The power button has damaged.
3. The cable that connect the system board to the power button is not functioning.

Please tell me how can I check and verify that the system board is damaged or still working fine. How to check that the power button and the cable are functioning. Or any other things that I have overlooked.