A big “duh” on this one! I had been excluding applications like cleanmgr, Task Manager, etc. one-by-one (those I could find, anyway) in the Behavior Shield and File System Shield and, but Avast was blocking many more, including IE (which didn’t respond to the exclusion), so instead in both these shields I checked the entire Program Files folder (x86, too) and the entire Windows folder (and I made sure that the System 32 folder was checked!). Now all are working. I haven’t checked on the black screen at boot-up yet, but I will at next boot-up.
Hopefully this fix will work for others with the same issues.
My only concern is if I’m in any potential danger from all these exclusions.
I tried a boot-up and the black screen is gone. However, something strange happened–the computer froze up (couldn’t access Avast or load web pages), so I powered it off and re-booted. Any ideas what happened?
In the meantime, I’ve gotten paranoid and undid all the changes. Hopefully Avast will come up with a permanent fix.