Thanks, but not at this time. The system does not need scouring with such intrusive tools, only one of which I'm familiar with anyway, since it is not exhibiting any unusual behavior or errors outside of this single avast! glitch -- and glitch I think it is, since avast! becomes unresponsive on popping up this particular warning.
What I've done instead is uninstall avast! completely, scrubbed the registry of it as thoroughly as I'm able, scanned the system with several other security apps and rootkit scanners -- which scans all came up clean -- defragged fully, wiped the free space, and re-installed avast! (free). After installation, boot-time scan, and manual scan of the Windows\Installer folder(s), which is where the claimed problem file was, results remain clean.
If the popup or anything similar returns and avast! remains unresponsive, I'll consider deeper measures, but I don't want to mess with a system that is in all other known respects intact.
THX JLJ