[Kristoffer O]

That is the general response to pirated copies of the game. If you bought the game from a legit reseller try to reinstall and use the CD-key from the manual. If you don't have the manual you probably got scammed by an unscrupulous reseller that deals in pirated copies.

Make sure you have a legit version of the game, especially before signing up in an MP game. Pirated copies will not work with more recent patches, and the average MP game lasts long enough to see a patch while the game progresses.