Whatever happened wasn’t harmless. It trashed my OS installation!

Here’s more info based on further research. When the initial Avast quick scan hung, it was scanning, or at least this is the file shown being scanned,

%APPDAT%.…\Start Menu\Programs[b]explorer[/b].lnk.

According to MS Security Essential encyclopedia, FakeVimes creates entries under some .lnk name.

Guess what is running under explorer.exe? Comodo’s Cmdagent.exe! Sorry, but I am seeing a pattern here that Comodo and Avast have co-resident problems and it appears those problems might have escalated to the point of causing irreversible damage.