Thermal compound is necessary to keep the cpu cool. Running the computer without cleaning off the old compound and replacing it with new isn't going to work. The computer will overheat and shut down. There are simple guides on the net explaining how to do the job properly. Why you assumed that the cpu was burnt out or that you needed a new power supply I can't imagine. This is working out expensive considering that the computer was working OK before you started. Pause for reflexion before you buy a replacement motherboard too.
