Yesterday, out of the blue, I got a Flag action from Windows 8.1 telling me that my Avast subscription had ended and that I was no longer being protected. I knew this was not true as my subscription was valid until 2016. I tried putting my license file in, but the virus kept invalidating it. I did an Avast Quick Scan and it found 4 high threat viruses with 2 of them being root kits. I, then, tried to repair Avast Internet Security, but the virus was still there.
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Finally, I uninstalled Avast, installed Malware Bytes, removed a registry malware, downloaded Free Avast from FileHippo, installed free Avast, scanned my computer without finding any malware and then used my license file to upgrade to Avast Internet Security.
It was a pain, but uninstalling Avast and re-installing Avast Free seems to have done the trick. I did use CCleaner and Wise 365 to clean my registry after the uninstall.