Thanks for the tip. Unfortunately, I won't be able to try it out for you, as I just got told another solution that works, and I don't want to rock the boat!
Apparently, the upgrade has a habit of hiding some of the old 2000 accounts (especially if they're called "Administrator"). So Ctrl-Alt-Del twice on the login screen will give you the old 2000 style login where you can enter the old login name. And that worked well. Then, a little bit of registry editing convinces XP to show the hidden account on the welcome screen.
If anyone wants to know how, then this link is what I used:
http://www.mydigitallife.info/2006/0...in-windows-xp/
Thanks for your help.