I have a doctor client who bought a high-end (for the time) computer running Windows Vista. Later she bought and install the Windows 7 upgrade, particularly because a medical training site she deals with requires it.
This week things went wrong and she needed to re-install. Got Vista on and put in the Win 7 DVD. "Windows 7 upgrade requires at least Vista Service Pack 1".
So I told her how to go to Microsoft Update and to keep going back until she was offered a Service Pack.
Well, 97 updates and a couple hours later Windows Update said, "Service Pack 1 is no longer supported and cannot be obtained from Windows Update".
It did not offer Service Pack 2 because Service Pack 1 is a prerequisite to Service Pack 2. Service Pack 2 is not cumulative.
Of course, Service Pack 1 is still available from the Download Center, but by this time a normal user is totally confused.
Perhaps Vista is such and embarrassment Microsoft is trying to get everyone to junk every computer that ever had Vista on it.