Thanks for your reply.
Restarting hasn’t fixed the issue.
The previous AV was Windows Defender. As Avast was installed, it turned itself off and accommodated Avast.
There is no way even to get to the frozen Avast window except by Alt+ Tabbing. It’s in the foreground, and it’s there to stay until you agree to the ‘smart’ scan, which it won’t let you do.
I guess it just expected me to ‘choose’ that option. But I wanted to configure it before running it- which should be normal.