Well, since I appear to have accidentally selected a trial version of Internet Security, which replaced AVAST! Free Antivirus, I uninstalled AVAST! and then downloaded and installed AVAST! Free Antivirus from scratch. Now when I launch the program, the user interface is the free one, instead of the Internet Security interface I usually get.
I believe my problem is solved. I guess the point is to be careful when dealing with updates NOT to check the free trial of the paid AVAST!
Thanks to DavidR and Para-Noid for your help.
