I dont really know who to contact, but I've known of this happening to people who bought a game from someone else. They would sell the game for cheap, even though it was registered on a different steam account (to prevent theft in a way). So if you bought that from someone else other than steam or a store brand new, you cannot do anything about it unless you buy another key. Thanks to steam and Iwnet not only can you not re-register it if you bought it used, but they wont even stop the hackers and bad connections that are constantly problematic. Sorry to hear about your copy of MW2 though, and I hope you can fix it somehow other than buying another copy of it.