Frame rates have dropped dramatically

I'm not sure what's going on, but games that I used to run easily have become almost unplayable. The frame rates have dropped to the point where things are more frustrating than fun. My hardware setup is in my signature. I've updated to the latest Nvidia drivers. Oblivion used to run great. No more (in fact besides being slow it now crashes after about 5 minutes). Bioshock almost becomes a slide show if things get busy on screen.

Any tips on what to look at to fix this?

Thanks.

Thanks, but I'm hoping there is something a little less drastic to try first. Reformatting is one of those "last ditch" fixes. As far as the PSU is concerned, I wonder if there is a program out there that can test that. I'll have to look.