Thanks for your reply. I wasn't signed into an account. I've seen the problem over the last two days only. I can't remember having done anything to trigger the problem or seen any "funny" messages or been on any strange websites.
However, I wondered if my system had had a hissy fit and decided to do sfc /scannow and this appears to have fixed the problem. There were apparently some errors in the file system which have been repaired. I don't know how to read logs so I suppose that I will never know what the problem was.
I have read the articles you linked to and changed a couple of settings which also may have helped - thanks very much.