COD4 - Backup Multiplayer Data

For those want to backup their Multiplayer data, here is where your files are located.

For Windows XP & Vista:
C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\YOURCOD4PROFILENAME

For Windows 7:
C:\User\YOURPCNAME\AppData\Local\VirtualStore\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\YOURCOD4PROFILENAME

Within the YOURCOD4PROFILENAME folder, there are three files:

config.cfg
config_mp.cfg
mpdata

The config files are storing your player settings (Keyboard, Video, etc). The mpdata file is the most important file which stores your game achievements/unlocks/rank.