I gave my friend my Call of Duty disk, but now I feel the need to play. I e-mailed him and got the CDs for one night, and installed it using the old serial code. However, when I tried opening the Multiplayer portion of the game, it said something like: This serial number is already in use' or something like that. I really want to play this game, but my friend won't uninstall or anything. So, what should I do?