Thanks for the tip.
In a nutshell, Microsoft installed Windows 10 on my Windows 7 PC without my permission; that is to say, I clicked the option NOT to install it when offered and the installation went ahead. I have since discovered that this is not an isolated problem.
To start with, it looked as if the new OS was working fine. It took several reboots to get into its stride, but eventually, after a day or so, it would boot without an error message. Time to do some work.
I tried opening a few files and slowly discovered, to my horror, that Windows 10 had converted ALL my files to "read only". More internet research. Yep, I was not the only person this was happening to. I tried resetting the permissions on files and folders, but W10 kept setting them back again. Eventually, I found a complicated work around which enabled me, slowly, folder by folder, to reset permissions on all my files.
And then, a day or so into the whole process, the computer just refused to boot. All I got was a black screen. Completely dead. Tried all the usual things. Safe mode was not available. The BIOS was unresponsive.
So I pulled the emergency cord and used Acronis, my backup software, to reinstall an image of the system as it had been back in July. Success! Except that now, every three days, Microsoft is nagging me to upgrade to W10 again and it won't stop! I can't turn the upgrade countdown off, I can simply postpone it by three days. At the end of that time, a countdown window appears saying "59 minutes until we install Windows 10". If I were not to catch it in time and tell it to wait another three days (you can't tell it simply not to install), it would start the whole nightmare all over again.
As I have researched this online, I have found many more people all over the world whose computers have been hijacked in this way by Microsoft. They don't want Windows 10 but they can't stop the juggernaut. One American I came across has been incensed enough to take it up with his congressman!
As for me, I am teetering on the edge of a decision. I seem to have four options, and I can't decide between them at the moment. (1) Let the upgrade go ahead and see what happens with fingers firmly crossed but expecting the worst; (2) Buy a completely new PC with a fresh install of W10 and expect it to work; (3) Reinstall my Windows 7 system from disc, lose all my programs and settings and condemn myself to reinstalling everything; (4) Abandon Microsoft and buy an iMac and spend ages learning a new system.
You see the problem?