http://news.bbc.co.uk/1/hi/technology/3994949.stm

Up on the BBC News website, they've got a little article on Halo 2, more to the point they've had a few words with Errera, the dude who maintains halo.bungie.org, he says:

"It does not feel like a game release any more. Somebody told me this was the biggest single release of any product in Microsoft's history."

I guess he's never heard of a little product called Windows then?