Well im my words it basicly going to be halo 2 PC but its only coming out for the new Windows Vista.
or as they put it:
As the name suggests, Halo 2 Vista will be brought to PCs with Windows Vista only. Of course you can always purchase Halo 2 for your Xbox or Xbox 360. There are currently no plans to bring Halo 2 to Mac, PlayStation 9 or the Commodore 64.
Some sweet pictures (dont forget to look at the resolution its amazing)