I’m running Windows 2000 with a couple of different user logons. As administrator, Avast comes up fine and the registration key works. When I try logging on as another user, Avast gives the message that there is not a valid registration key. I try entering the key, but it tells me that I need to log in as Administrator. Administrator logs in fine. Any ideas?
Are you typing in the registration key number? If so try copying and pasting the number from your original e-mail notification letter to the form and see if this works. 
Maybe the problem is the rights to access both avast folder and Windows Registry keys of avast. For the first one, just allow permition to all users on Windows Explorer (right click the folder > sharing … I suppose).
For the other, well, let’s wait for your first tentative, let us know if it works 