I am also having problems with the new version 18.1.2326. Read my post," after 20+ hours still only 44% done, this was after a re-install ".
I think maybe we both are having problems cause the OS is the old Vista. Right now I've had it scaning for 30+ hrs and its only 86% done. Today I ran the same version on my notebook w/windows 8 and it took around 2 hours.
Like you I've used Free Avast for over a year with no problems so I do not think the latest version is compatible with Vista.