Call of Duty help (Getting it to work, not in-game)

[So Many Kinds of Bagels]

I bought Call of Duty for the PC off of someone on another forum. The CD key and everything worked fine (I also got Halo off of him, which worked flawlessly). But when I double click "Call of Duty Multiplayer", and Call of Duty starts up, I am almost immediately confronted with a "Call of Duty has encountered a problem, and needs to close" error (the one that has "Send" and "Don't Send" as options). The single-player works fine. Google didn't help me, it only bombarded me with useless forum topics about proceeding in the game.

Thanks.

[THON]

Well, usually when error messages like that shows up it's because of your computer. I'd suggest rebooting, reinstalling, and in the worst case scenario if you're DESPERATE to play this game reformatting.

[So Many Kinds of Bagels]

I tried rebooting. I'll try reinstalling. It's just odd because the single-player works fine.