Have the same problem after I had installed trial version of avast internet security.
Never had it before.
I am afraid, it would be better to change antivirus rather to reboot my computer every time or to reinstall windows, etc,etc.
On Day 2 now of no Avast Internet Security and Win Start Menu and Cortana are rock solid, work every time.
Is Avast ever going to address this?
I have Avast installed and both Cortana and the start menu work without a problem.
It isn’t a problem for most. As to day 2, yesterday was Sunday and today is a holiday.
Hope you’re enjoying the holiday. 
So has this problem been corrected yet? I did pretty much everything recommended by BOB all ready if you read my OP. The problem seems to be a combination of the Antivirus and the new windows update. I would like to be able to use it again, but I’m not putting it back on until this issue is resolved.
Same thing happened to me. Just upgraded my Avast for the first time, and after the required restart, Windows Start no longer works. I have to launch apps via Program Files folder, which is so annoying. WTF.
I guess AVAST doesn’t care if I let my subscription run out in a few days. No one seems to be addressing this issue, and I’ve tried installing the latest available version with the same results.
Well the current version seems to have fixed my issue. I’ve been using it for several days now without issue. Would have been nice to get a notification here when a new version was released that might have fixed the issue, rather than me having to check back from time to time and try an install. I guess AVAST doesn’t do stuff like that anymore.
New releases are always announced here:
https://forum.avast.com/index.php?board=2