I'm not sure if this is a hardware issue, but it's my first guess.
I'd recently been playing a few games on my PC (Call of Cthulhu and Pirates!) and they were running fine. Then, all of a sudden, for no apparent reason, the graphics in both games got really choppy, lowered fps and such. It's gotten to the point where neither game is really playable and it's quite annoying. I ran the DirectDraw and Direct3D tests with DirectX and they were fine.
HD: 40GB + 10GB
Creative Soundblaster Audigy
Nvidia 6600 GT
Intel Celeron 2.8GHz