While experimenting with the font size (control panel/display/settings/advanced) I set it to 1.5 times the normal setting, to 144 dpi. I also experimented with the resolution.
I changed the resolution back to where it had been (800 X 600) but I can't change the font size, because the "apply" button on the page that does that is no longer visible, even in full screen mode. The "advanced" button on the preceeding "settings" screen is half hidden but useable.
The system is about 5 years old and has a Samsung 19 inch flat panel monitor. I have had no significant display problems until this one, today.
Thanks for your help!