I've never logged in with my 'administrator' account, I've only used my user admin account for these uses, and use my power user account for normal use. I'm running XP Pro fully aware, and capable of using the real administrator account for everyday use if I want, I just prefer to never use it except for system recovery. I would never give another person administrator rights on my computer however, I don't want them to install software without my permission, or even allow something malicious by their ignorance/stupidity to take over my computer.
This entire issue would seem to be how Avast stores the registration key if it can't read it from all user profiles once already registered.