This seems to be an ongoing problem with no clear solutions as far as I can see in this forum. I'd like to post it again, in case I missed something. I ran into this problem helping out on my friends computer.
My situation: I was working on a system with two user accounts. Both had Administrator privileges. I ran into the "not enough user rights" error message trying to update the current user (I'll call it user A). After looking at the forum I changed user A into a Limited account and logged in as the old user (I'll call it user B). Voila! I was able to update Avast.
Here's where the problem resurfaces...I wanted to remove user B altogether and install Avast into user A. Firstly, I logged into user B and I was able to complete uninstall Avast from the system. I rebooted to finish uninstall. After restart, logged into user B and changed user A back as Administrator. I logged out of user B and logged in as user A. I then deleted user B from the system. So far so good. I rebooted for good measure. Now, with only one user account (user A) as Administrator, I thought I'd have no problems installing Avast... Well, I was wrong. I still get the same "not enough user rights" error that I started with originally. I've noticed that in Regedit that in HKEY_LOCAL_MACHINE/SOFTWARE/ALWIL Software still exists and the system will not let you delete or change permissions for that folder/key.
Suggestions?