After reinstalling Avast!, I ran a boot-time scan (which took 2hrs. 47 min. to run), during which time Avast came up with hundreds of "Cab archive is corrupted - Error 42127" messages
[b]files that can not be scanned[/b] for reason given by avast is this forum most frequently asked question, so plenty info if you forum search

files that can not be scanned is normal and it does not mean they are infected, just that avast could not scan them, no more no less
avast may say corrupt archive if it can not unpack the archive and scan it…nothing to worry about, those files will be scanned by the realtime protection when whatever program use those files access them