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If it's not I think Bungie needs to make an official announcement about it because there are people already saying that they will wait for the PC version.
The last man on Earth sat in a room, there came a knock at the door.Service Record
Bungie hasn't confirmed or denied the answer to your question.
There is no reason for it to be. Those people will be waiting a long, long time.
If Halo 2 is going to appear on the PC, it will most likely be released a year or more after the release of Halo because they would not want to take away the Halo 2 sales on the Xbox.
it'll come....................................eventually