I've played COD4 a lot on Windows 7 64-bit, and have never experienced what you're describing, so I don't think it's a Windows issue.
I'm running it on a C2Q, 8GB DDR2, Asus P5Q-E and crossfired 4870's. I don't think the processor/RAM/MB would be causing this issue, so I'd be looking at your graphics drivers as the most likely suspect. Which version are you using, and have you tried updating/rolling back? Sorry I can't suggest ones to try, but the last Nvidia drivers I used were in XP on a 6600GT, so I'm not too current I'm afraid!