Fixed it! Or, rather, I got a pro to fix it. Found more than 1500 bits of advertising and other similar stuff, much of it "malicious" and dangerous on my computer and also several viruses that had got past Norton in spite of it being fully subscribed to and fully up to date in terms of its virus definitions. I now run AVG and Ad-Aware every morning - thank you to those who have published the relevant advice here on PPruNe! Fingers crossed!