When good programmes go bad...
I came to the computer freshish from a night's semi-sleep - I spend a lot of the night awake and worrying - and decided to try one last thing. My computer was, after a system restore to the point immediately after the soft reinstall, running very very slowly and glitchily, but at least running with icons and start menu that I could actually access. Clearly doing a system restore - even one that could only go back a few hours - had got rid of accumulated programme crashes.
I noticed that ZoneAlarm, my firewall, was occasionally flashing up a message saying that it was trying to install part of itself and giving me the option of cancelling - while disappearing too quickly to do so. I managed to get up Windows Change/Install Programmes and removed ZoneAlarm from my system altogether; instantly everything else stopped hanging. I then downloaded and reinstalled ZoneAlarm, which is now running normally, as is everything else.
What have I learned from this? That no programme is wholly my friend and that trial and error and a constant process of eliminating possibilities are the way to go when there are computer problems. Is this the beginning of wisdom? We shall see.
My thanks to everyone who has been helpful.