According to the Gamerscore Blog…Halo 2 will be released for Vista. This gives us a good indication of timeframe now anyhow.
Halo 2 is making its PC debut on the Windows Vista platform. The game will have both the single-player campaign and multiplayer experience of the original, as well as the additional maps offered in the Halo 2 Multiplayer Map Pack. Gamers will also have the opportunity to build their own multiplayer levels.