|Boot to BIOS and look for internal temperatures. High temperatures would indicate a serious cooling issue. If in changing out the cooling fan, you removed the heat sink from the CPU, then all of the old thermal compound needs to be removed and new compound needs to be reapplied correctly.|
Also, are you sure that all connectors/wires reseated properly during reassembly?
You have to be a little bit crazy to keep you from going insane.