Halo 2 PC
From Halopedia, the Halo wiki
Summary
Bungie Studios, along with a Microsoft Game Studios, is currently working on converting Halo 2 for use on the Windows Vista operating system. No Mac or XP version is planned as of now.
It has however been confimed that there will be a map editor that will be made by Microsoft (in cooperation with Bungie Studios) and packaged with the game. The extent of the map editor's capabilities have not yet been made available.
Halo 2 for Windows Vista will also not link with the Xbox Live™ network. It is totally incompatible; however Bungie has stated that it will have networking features of its own like Halo PC.