EVGA Geforce 8800GTS (G80) 640mb Problems

I recently reformatted, and I currently have gotten everything reinstalled properly. However, many of my games will no longer function at all when I try playing them. Dawn of War, for example, will show up a black screen, and my system will get so slow to where I can't do anything but hard reset the computer. Also, in some other games, I get ridiculously low FPS in them, where before I formatted they worked like a charm. It can't be the PCI-E issue my 965P-DS3 has, because I have already fixed that. This is very annoying because I can't play any of my games, and the ones that I can play are having horrible framerates.

Does anyone have any suggestions on what do to as far as diagnosing my video card and/or other things?

Thank-you.

Edit: Here's my system's specs, I almost forgot lol.

Hmm, this was weird. Yes, I have tried reinstalling the drivers, but before it didn't work. Today I did it, and now my games work.

Strange...

Anyways, thanks for the help