I've just cracked the problem! Went to Control Panel/ Ease of Access Center/ Make the Mouse Easier to Use/ Make it Easier to Manage Windows. “Activate a Window by Hovering Over It With A Mouse” was ticked (I don’t know how!) Unticked it, and my windows and mouse actions are back to normal. Why anyone would ever choose this option really beats me!
Thanks for your prior help, Spurlash2.