Jock1e - Thanks for the suggestions and the fact it may be the screen, not the mouse. We will try your suggestions and see how it goes. Incidentally, I use CCleaner (and do so on three other computers for several years) (never used Tune Up Utilities) and have never had any problems with this computer or any of the others.
Update the driver - for the screen (PC has a separate graphics card, so presumably update the graphics card driver??). Or try to update the mouse driver??