Before you go and scroll down to the comments section, yes the Xbox version of Halo 2's multiplayer but as of February 15th 2013, the PC version will be disconnected from the Games for Windows service interlinked with Xbox Live. On April 15th in 2010, the Halo 2 servers for Xbox Live were officially shut off. During that summer, dedicated fans of the 2005 release clammered to their 4:3 TV in hoping that their connection wouldn't give out as the servers had been officially shut off except for those games still connected to an online match.
As less than 50 players have peaked being online at the same time for the PC version, 343i, who is now in control of all Halo information online, decided it was time to pull the plug. As I never got to play the PC version, I'm not saddenned but nastolgic to go back and play through the campaign on my 360 again.