DavidR and those who assisted me.....
You were good enough to help so I feel I should update you.
Avast Team did not respond. I didn't want to waste time on a clean install of Avast only to find that the problem still remained, as it did with the repair.
So I uninstalled it and installed ClamWin, but was later told it wasn't enough.
I have Comodo Free Antivirus too now. How effective that is I'm not sure - read differing reports.