Did you make sure your power supply can handle the new motherboard and whatever else you added to it?

Disconnect all hard, optical drives and fans except the CPU fan.
You want to check run the motherboard, RAM and video card preferably on-board video.. A cooling fan draws ~1/2 an amp. If you count all the current that is drawn its substantial.

Edit: Make sure your CPU heat sink/fan is properly attached. It is possible you are experiencing thermal shutdown. I assume you removed the heat sink/CPU to reinstall it. You have to completely remove the used the thermal paste heat sink and CPU. They need to be clean as a whistle. Apply a small drop of new thermal paste to the center of the CPU. Chances are you separated the CPU from the heat sink with out using fresh thermal paste. Bad move. Thermal distribution is very important for keeping the CPU at a working temperature.

Also you have two different motherboards which require different divers. Chances of the PC booting with the drivers from the old old motherboard are slim to none.

You didn't say which OS you are using. Sysprep for XP seems to do the removing old drivers in many cases.

Jeff

also make certain both power connectors(20\24 pin & 4 pin) to motherboard are connected.