. . . and why did it suddenly appear on my PC? (As a series of pop-up messages and warnings and invitations to register it.) Is it an official Microsoft program, or cleverly-disguised malware? My PC has Vista, plus Norton 360.

It's malware, get rid of it by initially running Rkill, then MBAM: Rkill will stop all running processes, both legitimate programs and recognised nasties. By doing so, it will then allow you to run MBAM to remove the infection(s). Before running it, disable any anti programs that are running as they may see it as a threat. The program doesn’t delete anything, it just stops processes from running, corrects the registry and restarts Windows Explorer. It will take less than a minute to run and produces a log file showing which processes have been stopped. If you can’t download it, save it to an external USB device using a different computer, then plug it in to the infected machine. It is small and doesn’t need installing. Now run, say, Malwarebytes Anti Malware to remove the infection(s). Don’t restart the computer until the infection has been removed, as any processes that Rkill stops will be restarted when you restart the computer, including, of course, any nasties that haven‘t been removed. Read this, then download the free Rkill here http://www.technibble.com/rkill-repair-tool-of-the-week/ Free MBAM here http://www.malwarebytes.org/