What a clear and helpful answer. I am going now to buy the disk and will put your plan into action as soon as someone is free to help me. THANKYOU!

dear dalchina, thanks VERY much for your quick and clear response. Dont worry, no question is too obvious for me.
I have no system restore point and nothing with which to restore to previous point....

If I open my windows 10 file explorer " this PC downloads", and want to see thumbnails in extra large size, this suddenly no longer works.
All the other sizes still work. Extra large is highlighted...