Error when trying to join a game on HaloCE

[sarachan]

Hello. I recently downloaded HaloCE. Any time I attempt to join a multiplayer game from within HaloCE, it gives me the loading screen and then an alert saying "The game has closed down." And yes, I'm using a CD and key from a legitimate copy of Halo. Does anyone have any suggestions as to how I might go abouit fixing this?