Changing Power supply

Check the power connections to make sure they are snug. Also, was the power supply used or new? If used, was it working before installation into this system? Do you hear any strange beeps upon turning on the system? If yes, check the beep codes on the motherboard manufacturer's site. Also, check to make sure video card and CPU is set snug in the slots. Since you took off the CPU, take it out and make sure you didn't bend any pins and reinstall it.

HTH

A common thing that is often missed is the voltage selection switch.

If the switch is set to 220 and you are on a 120 line, the power supply will not develop enough output power to run the PC.

Another thing to check for is bent pins on the CPU.
CPU not seated and latched into the socket.
RAM loose.
AGP card not seated.

Depending on how you handled the CPU and cleaned of the dust, it is possible that the CPU was damaged by either physical stress or static electricity.

Chas

Make sure all power connections are re-connected and tight with no bent pins. There are sometimes multiple connections fromt the power supply to the motherboard (ive forgotten to reconnect the smaller one a couple of times - easy for it to wind up under something so you dont see it and know its loose - follow each wire to its end and make sure it doesnt belong somewhere - to verify everything tight - disconnect and reconnect Firmly. Also agreed with prior answer - check voltage - new ones i find often come on set on 220 lately. This can cause the machine to act strangely.