I'm running into a similar problem with an XP-over-XP reinstall
(google for my recent thread).
If you bought a retail-upgrade version, contact Microsoft, they
give free install-related support for non-EOM customers.
Unfortunately, I'm using OEM.
There is a catch though - their web site wants your product ID#,
which you probably can't get to. You might have to call them on
the phone. I don't have the # handy.