Initially I ran Sophos anti-virus scanner and Malwarebytes and neither found anything malicious. I have also run AdwCleaner and ComboFix (they did find and remove things) and used CCleaner to clean the Startup entries and the Registry. The PC was a bit slow at times, but does run reasonably well.
Can you offer any clue as to what sort of thing this might be, or where I might be able to find/eliminate it?