Try disabling any unnecessary background applications. Make sure your graphics drivers are up to date, too.
Also disable vSync in the options. If you still get frame rate issues, you might try the following steps in the Nvidia Control panel:
1) Click on "Manage 3D settings" in the left panel, then find the profile for Modern Warfare 3 in the scroll bar that should appear in the right panel of the new window.
2) Assuming you're not using multiple displays with your GPU, go to "Multi-display/ mixed GPU acceleration" in the MW3 profile settings. Select "Single display performance mode" in the opposite pane and click "Apply".
3) Next, go to "Power management mode", select "Prefer maximum performance" in the opposite panel and click "Apply".