this should tell you
http://www.cod4central.com/forums/cod4-pc/a-warning-to-all-pc-cod4-gamers/0/well first save your payer profile!
C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles
this is where it will be located. its best off to copy the whole players folder and paste it in my documents or something.
now you can uninstall cod4 while still having your profile intact:)
when you've reinstalled, just move the players folder back into
C:\Program Files\Activision\Call of Duty 4 - Modern Warfare
you'll have your profiles all there for you

good luck!
posted that somewhere else aswell.
just copy the whole "players" folder and paste it onto a usb pen or something.