|It's good to know a few things:|
"Bad" motherboards are rare, unless they went bad from mis handling, electrical shorts (in the case) or electrostatic discharge.
Is it possible that you bent a pin(s) on the CPU? Are you SURE that the CPU cooler is installed correctly?
(Modern CPU's, with their very small surface, take just seconds to overhead with no sink properly installed.)
Are you SURE you have the proper kind of RAM, and that it is good? Do you have another computer that you can "try" the RAM in?
Carefully check the "manual" to be sure there are no CPU/RAM voltage or speed jumpers to set.
I would DEFINATELY try "resetting" the bios. Refer to the manual, there may be a jumper. If not, what ALWAYS works, is to remove the "coin" battery for a period of time, to be sure, wait 15 min or so. Do this with power OFF.
Also, hook this up as follows:
On a SAFE TABLE, NOT in the case, hook up ONLY
The power supply, motherboard
RAM, CPU, and VIDEO
and the keyboard.
THAT'S IT, nothing else.