I an being forced into this mess as well. I needed to perform a System Restore on a Win 7 box, and Avast popped up saying "The trial period could not be activated. In order to start receiving updates, the trial period must be activated." Dandy, except I was happily running Avast Free. There appears to be no way to have Avast keep working *without* activating the trial. Avast was sitting at version 8.0.1483, claiming an upgrade to 8.0.1455 was available. As far as I can ascertain, this upgrade does not exist, nor do any of the other Avast installations report that such an upgrade is available.
I then uninstalled Avast, downloaded the latest version, and reinstalled Avast Free. Well, that worked except that Avast refuses to register for continuing free protection. The only workable option is to register for a "free trial" of the pro version. I did, then set the computer's clock forward to extend past the trial period. Trial expired, can no longer revert back to free version, pay up sucker. I then went through the cleaning, etc. steps listed in earlier threads. Reinstalling Avast Free is not an option - it wants payment.
This whole process would have been more frustrating had it not played out in a virtual machine (we use VMs for instrumentation control; our corporate subscription to Symantec would work fine, but having a lighter-weight virus scanner is desirable). Restoring the entire VM from a backup took a minute or so, leaving Avast Free operational.
Based on this experience, I certainly would not want to rely on the "free" Avast to provide reliable, long-term protection for anything other than a disposable virtual computer.