I love CoD4 but have become quite bothered with a couple of the challenges so I decided to seek out a hack. I was able to get 46 of the challenges completed on my own but couldn't be bothered to get the rest.
I do not recommend doing this if you haven't done a considerable amount of the challenges on your own or you're going to take away a great part of the game.
Anyhow, here goes...
I tried all the YouTube tutorials but to no avail, as well as another tutorial I found but neither allowed me to successfully hack the ranks. I took what seemed appropriate from the two and fiddled around and I got it to work. Here's what I did:
1. Go here and download the "mpdata" file - http://www.megaupload.com/?d=AGZQZGLN
2. Go to your "players" folder and copy it to a location outside of your Call of Duty directory. (i.e. - My Documents). This will be your backup copy just in case you screw up. Default location for the "players" folder is C:\Program Files\Activision\Call of Duty 4 - Modern Warfare/
3. Open the Start Menu and click "Run".
4. Type "regedit" and click OK.
5. Navigate to HKEY_LOCAL MACHINE > SOFTWARE > Activision > Call of Duty 4
6. Look to the right box and you'll see the second row is your "codkey". Right-click and select "Modify". Now enter DL2J8PY44Q22GE4888D2 as your new key.
7. Click OK and exit Regedit.
8. Navigate back the original "players" folder again and inside it you'll find your profiles. Open the one you want to upgrade and delete everything inside of it. (Make sure that you have completed step #2 first!)
9. Move the "mpdata" file that you downloaded earlier into the now empty folder.
10. Open CoD4. (You may need to reset some settings such the display resolution, etc.)
11. In the game, go to Options > Multiplayer Options > Enter Key Code and re-enter your key that came with your game.
12. I've read in a couple of places that you will need to play for at least 20 minutes before exiting the game, so I played for around 25 minutes. Not sure if it's necessary though.