1.Download this [ Register or Signin to view external links. ] file.
2.Go to Start -> Run-> Type regedit -> Hit Enter.
3.Go to HKEY_LOCAL_MACHINE -> Activision -> Call of Duty 4
4.Double-click codkey
5.Save the key in notepad
6.Replace the key with: DL2J8PY44Q22GE4888D2
7.Press OK and close regedit.
8.Navigate to your Call of Duty 4 profile folder (typically C:\Program 9.Files\Activision\Call of Duty 4 Modern Warfare\players\profiles\your_user_name)
10.Back-up and then remove all the files in your user name folder.
11.Place the mpdata file you downloaded in your user name folder
12.Open the COD4 MP game and check your rank. You should now be level 55 with all 13.challenges, guns, and every camouflage unlocked!
14.Finally, in the game go to Options -> Multiplayer Options -> Key Code and type your original cd key you have saved in notepad.

There you go, you now have everything unlocked, Enjoy.
This Method was made by Karma Rouge I belive