As it turned out, I remember downloading a "fish tank screen saver" some years ago, which never worked, and which I promptly forgot about. However it seems it had something to do with allowing a trojan horse infection to take root later. Thankfully it was not one of the most serious variety, such as a backdoor Trojan. I now know about the risks of "free" software. I was quite anxious at first to have my problem fixed, but the time spent lurking and reading the articles before posting really did pay off. I can now say and I owe it all to nunped and the friendly staff here at malwareremoval.com.