If the popup or anything similar returns and avast! remains unresponsive, I'll consider deeper measures, [b]but I don't want to mess with a system that is in all other known respects intact.[/b]
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.
you did all this and call running the requested tools for messing with the system. ::)

there is no mess, attach the logs and they will be analysed by certified and trained malware removers…
if you surf the viruses and worms forum section, you can see there daily work