If Automatic Update doesn't offer it to you. Then I guess Microsoft doesn't see your install as a candidate for the update. Then as you pointed out. It looks like the IT version at present is the only alternative.
I know someone with 4 XP computers and only one offered it. He did add it to the other three with no problems.
He may have used the downloadable IT version as I would not want to do about 3.5 hours of download time myself.