The way to do this is fairly easy as all that is required is to save your players folder and place it somewhere else on your computer or on an external source and reinserting it when needed.
IF YOU HAVE WINDOWS UAC DISABLED
The players folder is located in the ActivisionCall of Duty 4 - Modern Warfare folder: for instance mine is located at C:Program Files/Activision/Call of Duty 4 - Modern Warfareplayers. See image below if needed.
Simply right-click and copy the whole folder, paste to your desktop (or other storage location or external source), re-install the game, then put the players file back where you found it.
IF YOU HAVE WINDOWS UAC ENABLED
Unfortunately windows likes to hide your players folder in one of its many notorious hidden folders. To access your players folder go into your windows users folder and open the Window's user folder that you use when you sign in. To make the next step easier for those of you unfamiliar with finding hidden folders I have screenshots. These were taken in Windows Vista Home Premium but should be fairly close to other Windows OSs.
Once in your user folder click the browser bar at the top and add appdata. for instance mine is located as you can see in C:users/chris/appdata. See below:
Once in the appdata folder rightclick in the appdata folder but not on a subfolder and click properties. See below:
Inside properties uncheck Hidden click apply and if prompted choose to Apply changes to this folder,subfolders, and files. This will unhide all the other needed folders and make it so you can see your appdata folder without doing this everytime. See below:
Once this is done go back to your appdata folder then click local, then Virtual Store, then Program Files, then Activision, then your CoD4 folder, and then there it is the players folder. Simply right-click and copy the whole folder, paste to your desktop (or other storage location or external source), re-install the game, then put the players file back where you found it.
BACK-UP FOR UAC AND NON UAC USERS
If you are simply wanting a back-up follow the above minus the re-install. It is preferable to save your back-up to an external source- be it flash drive, external HDD, webpage, CD/DVD etc.... as this will prevent the loss of your stats/unlocks should your HDD fail. Also you should remember that you will need to periodically back-up your players folder if you unlock new things (say golden weapons) as they will be lost should the unfortunate happen and all you have is a backup from before your unlock.