I've had COD4 installed for about 4 months without any problems. At some point, some security software was taxing COD4 so that everything was extremely choppy. I used a clean boot to turn off all my services and startup items that weren't microsoft related and it fixed the choppiness. But now I'm getting really low fps compared to before.
Turning the graphics down
Reinstalling the drivers for the video card
Updating the bios on my motherboard