Microsoft announced that Halo 2 for the PC will be released on May 8. The game will require Windows Vista, and is in development by Hired Gun, an internal Microsoft Studio working with Bungie. As quoted in this Geme Spot article
"Halo 2 for PCs will include 1,000 achievement points. The game can also be played with either the mouse and keyboard or the Xbox 360 controller for Windows, and will include a map editor. As stated previously, multiplayer gaming will be limited to like platforms; PCs will only be able to play against other PCs." The game will require Windows Vista and is integrated into Microsoft's new Games for Windows Live which it the PC equivalent of Xbox Live.