ExGrunt: Thanks for the advice. I don't need to use memory sticks with my applications, so these viruses must have got in from t'internet under AVG's protection screen. As they must have disabled AVG's ability to update then they must have been pretty new as that problem only appeared on my main machine last weekend. Furthermore a freshly downloaded copy of Avast did not spot them either - though it probably would have done once it had updated itself, but after its unsuccessful bootup scan the other day, it found nothing when it had updated and a further full scan was run. Fortunately for me a freshly downloaded copy of Malwarebytes did contain the necessary data and I expect the reason that it had a problem in installing itself was because the virus was (unsuccessfully) trying to defend itself.
On principle, I always use the freely available recommended antivirus and antimalware software. I wonder if I would have remained protected if I had paid for this facility from one of the big providers like Norton who so often come under criticism for being "bloatware" and slowing down the computers into which they are installed to a crawl.
P.P.