1. Open up Windows Explorer and Navigate to the following folder:
C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\Players\Profiles
(Or the appropriate directory where Call of Duty 4 was installed to)
2. Choose the profile of the stats you want to save by opening up the corresponding folder and copy the file named "mpdata" to a safe location. This file holds all your stats, ranks and custom kits etc.
3. Now that your "mpdata" file is safe, when you next have to re-install simply load up the game once it is installed simply copy and paste the "mpdata" file back into your profile folder. This should then restore your rank, stats and custom kits from when the "mpdata" file was last backed up.