What you say makes a lot of sense. I suspect it was malware, but it was connected with W10 somehow because it happened only after I installed it and stopped after I went to W8.

Probably true. Would that it were as simple as that.

I tried to contact Microsoft back then about the problems, but didn't find this forum. Then today I got another popup, a somewhat more insistent one urging me to upgrade now or postpone, so I...

I upgraded to W10 from W7 several months ago. In the succeeding week, W10 reconfigured my hard drive at least twice a day and, in the process, deleted all my files. Fortunately, I had a backup, but...