Hi Chris,
many thanks for your suggestion regarding the question I had posted in Chatterbank
http://www.theanswerbank.co.uk/ChatterBank/Question1222588-2.html
I downloaded Malware and did a full scan.
It detected some items that I had downloaded recently while trying to find a programme to convert doc. files to pdf. files.
These files have now been removed and it seems to have done the trick.
Thanks to others who gave sensible answers