Learned a good lesson last night:
1) deinstall avast before upgrading in the fast ring
2) after the install is successful, create a good restore point.
3) install Avast and see if everything ok.
In my case, as in others, Avast crashed Windows (or Windows crashed Avast if you prefer) - the UI wouldn't boot to the login screen. Used System Restore to get back before Avast install..
Looking forward to an announcement that the issues has been found and fixed
