What is the operating system? Vista machines have this thing (it's a bug) where if you install multiple users on a machine, you end getting the "blue screen of death" which isn't so bad as the black screen, by the way, but just as annoying.
Also, you'll want to have the CPU itself checked, to be sure you didn't fry something when you were running the machine without the thermal paste, which is what holds the heat sink onto the CPU, for one, and keeps the heat sink from overheating the CPU itself, for another.