When Microsoft updated to 10, a message popped up that Bitdefender was being removed. Overly aggressive, IMO. Anyway, it apparently removed part of Bitdefender, but there was still a Folder full of Bitdefender files.
I ran the uninstall tool. This takes quite a while, BTW, and it looks a lot like it is looping in unsuccessful attempts. I was pretty close to closing it out when it did actually finish. After a restart, Windows Defender is functioning.
So I guess the next question is whether Windows Defender is considered adequate these days or if should install the latest version of Bitdefender, or change to something else.
After settling on the first line of defense, then I guess it will be time to reinstall Malwarebytes, which remains non-functional except when accessing Chameleon.
When Defender was fixed, it wanted to allow some updates, which I did. No problems, except HP wants me to run the installation CD for the printer. Over the years, HP has been nothing but trouble with their software, so my inclination is not to do anything with that as long as the printer seems to have no issues.