Here's what I tried.
I went to
http://www.bleepingcomputer.com/malware-removal/remove-xp-antivirus-2008-20...
There I downloaded Malwarebytes' Anti-Malware program. I followed the instructions on the bleeping computer entry, and I think it's gone. No more pop-ups, no screen saver of warning.
Somewhere along the way my Photoshop was de-activated, but that's another story...