Hi Rednose,
thanks for the link and for your kindness. I am going to reinstall Avast 2014 which worked just fine. I have a lot of minor issues besides the one detailed above. For example, Avast 2015 takes almost 10 minutes to load when I turn on the computer (after everything else has loaded), Windows Explorer crashes frequently, Avast interferes with Truecrypt whenever I mount and/or dismount volumes, and so on.
Too many problems to try solving them. I rarely experienced so many bugs in one single program. Perhaps the 2015 edition is not really made for XP.