Thank you, BOAC.
"Curiouser and curiouser", said Alice!!
Very long and complicated saga of trying to download/install AVG - FAILED!!
Similar length process with Avast - success, sort of!!
Tried to update Avast - failed.
Changed to laptop - same result with AVG but managed by devious means to install Avast and ran it for boot check. Masses of 'nasties' and trojans -all deleted. Registered Avast and update completed.
Looks like the desktop will have a similar mess to clear and my reliance on AVG was misplaced??
Ain't pooters wunnerful??
Thanks all.