Computer will not boot.

I am troubleshooting a Compaq Presario 5000 desktop. When I turn the computer “ON”, it will sometimes get to the Compaq logo, on other booting efforts the screen is dark. When the Compaq logo does appear the computer is locked up. It cannot be restarted with a soft boot. I have installed the following good devices: A hard drive, a video card, a power supply, a RAM memory module, and I have reset the BIOS. This leaves only the system board as the culpret. Does anyone have a suggestion for correcting this problem?

Thanks