Backup your Multiplayer data!To backup your multiplayer profile is easy. All you have to do is navigate to the place where your Call of Duty 4 is installed and find your user profile. EG: C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\User Inside that folder find the files called mpdata and config_mp. mpdata contains all the xp and kills, headshots etc. config_mp contains all your keybinds and the names of your custom classes. Copy these two files and paste them somewhere else, or save them to a disc/external device. You can then re-install Call of Duty 4 - Modern Warfare. Then go into the game and make a profile. Then exit the game. Then find the same profile again on your hard drive and copy your files over the ones existing in C:\Program Files\Activision\Call of Duty 4 - Modern Warfare\players\profiles\User. After that enter game and it should work. It is also possible to have many accounts with the same stats and keybinds in this way.