dude idk it MIGHT be of ur video card remember MIGHT BE not is
anyways im not sure try reinsatlling agian if did not work check clock settings in ur vid card they might be underclocked by mistake check rivatuner if u have it if u dont download it CHECK UR CLOCKS
Yeah something doesn't seem right. I beliece MW2 runs the same engine as COD4. And I could run COD4 no AA at 1680x1050 on a 4670, which is slightly under a 9600GT. Something isn't kosher. So all the other games are currently fine, its just MW2? I would individually check each display setting and make sure something isn't cranked or dual GPU is on or something.
Try turning texture resolution, normal map resolution, and specular map resolution to high or even a lower setting. I had terrible lag to the point where the screen would momentarily freeze when running these parameters at the maximum setting. Turning them down to high solved the problem completely. I think there is some kind of bug that causes problems when these parameters are set at the maximum.
I am running a 9600GT also, 1440x900, E4500 stock(2.2ghz) and am getting 40 to 60 FPS most of the time with all settings at highest except those mentioned in the first paragraph. Aliasing is turned off, but I tried running with it on and the performance was still acceptable. You should definitely be getting better performance.
Try updating your drivers, and check if the system is overheating (clean out dust, etc.) Also check if some program is running in the background that is using a lot of you CPU or GPU power. One day I was having terrible performance and exited the game and found something was using 50% cpu. Restarting the system stopped this and restored normal performance.