Didn't you already reinstall your OS? It is not your PSU, keep in mind that it is pretty high end. I got an idea, try to download your drivers from AMD again. Link: http://sites.amd.com/us/game/downloads/Pages/radeon_vista-32.aspx
That should be your driver. ONLY DOWNLOAD ONE DRIVER AT A TIME! When your computer asks you to restart, restart it, and then download the other packet. Download only the top and bottom one, no need for the middle one. The top one gives you the middle one and more. Try that and tell me what happens.