I'm also running the free Avast free version 6.0.1289, and have been using Avast for many years. I'm having the same problem.
For the past several days--perhaps even a week--I've been getting the message that my registration will expire soon. I've clicked on the "re-register" link, which then flashes a message that says something like "retrieving information." A pop-up window then appears, which SEEMS to give me the choice between the free version and the paid version. However, the select button for the free version is deselected and gray, while the select button for the paid version is green, active and can be clicked. True, when I move my cursor over the select button for the free version, the cursor does turn into the selection hand. However, since that option is deselected, nothing happens when I click on it. I've restarted the re-registration process, restarted my computer, tried on a variety of days, but have gotten the exact same result each time.
It appears that Avast is no longer giving me the option of the free version, and I must pay to use it. However, in various places in the software, it still says I can opt to re-register for the free edition. Is this just a programming error on the pop-up "re-registration" window? Or, must we purchase the full version if we wish to continue using Avast?
Any help you can give is much appreciated!