Thanks very much for your prompt reply.
I was going to follow your suggestion, but now the interface won't open at all! I will restart my PC, and if all is well, I will continue with Avast One. If the problem recurs, I shall probably try another security solution, as it is worrying if a program designed to protect your PC cannot be relied on.
I have always used paid security programs previously, but my preferred one no longer supports Windows 7, so I thought that I would give the free Avast One a try, as I had read good things about it.
Thanks again for your help.