The most common mistake is an extra mounting post on the backside shorting out the motherboard.

Next is recycling a power supply. I can only guess what you need, but let's peg it at 400 or more Watts on the PSU.

Finally, I strip it down to just motherboard, CPU (I'll tell why I don't remove it in a sec) and the PSU. No RAM, boards, drives, etc.

I don't remove the CPU since the Athlons have a CPU core that can be cracked with removal of the heatsink, so I leave it in place.

If it powers up, then power off and add one stick of RAM. Power up (lots of beeps we hope!) and then power down. Add one part at a time until its complete or you find the bad part.

It's also possible the CPU FAN sensor isn't working, but the first test can tell that.

Bob