BOAC - many thanks - all sorted now.
Regarding the other comments - this is the first problem in a couple of years and is on a computer the kids use so, personally, I think they may have downloaded something that triggered this - but hey I am no expert. I have AVG on this computer and I am happy with it. I use firefox and google chrome as browsers. But, I agree, I don't think you can be 100% safe.
Thanks again.
HR