This is for BigGerm
Copy the player folder to some where safe maybe a flash drive or what not.
To backup in XP: Backup the folder called "players" in your COD4 directory C:\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\players\ To restore in XP: Simply place it back in the same place, ensuring the folder in players\profiles which should be your username, matches the text inside the active.txt else it will not work In vista it is...
C:\Users\"USERNAME"\AppData\Local\VirtualStore\Program Files (x86)\Activision\Call of Duty 4 - Modern Warfare\players\
In Windows7 it is...
C:\Users\"USERNAME"\AppData\Local\VirtualStore\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\
I hope this helps for next time. in the mean time just shoot the X on the screen youll get your rank back in no time LOL