Thank you Avast for the quick reaction and fix. Installed latest build this morning Swiss time, and now my issues with latest Win 8.1 Upd3 (November update) are fixed wrt Avast and also, ClassicShell (I presume the incompatibility with Avast was deadlocking something in ClassicShell).
Even though the last 2 days were a PITA for users with almost bricked PCs like mine, I don't think Avast is to blame - I rather have congratulations for you. Now, how did it occur that such a widely used product as Avast was not detected as impacted by Microsoft QA teams when they tested November update, is surprising actually.