I have a similar problem on my Windows XP (home ed, SP2) where I use several profiles (for the members of my family).
I managed to activate the Avast! in one profile, but whenever I log into any of the other profiles the Avast! wont start automatically and when I try to start it it states "Invalid license key".
Yes, I log in as Admin, Yes I have the correct time, Yes I have a valid licens key, Yes, I have checked that the services are set to automatic startup, Yes I have pasted in the exact license key and Yes I have sent the question to the Avast! support team 2 weeks ago and still have not received any answer...
What to do? I would be surprised if this program really wont be able to handle multiple XP users on the same computer.