Solved the issue. I have been using my Macs most of the time, so I forgot this. I ran across this thread in the Spybot Forum:
I ran Spybot as Administrator (by right-clicking the icon and choosing 'Run As Administrator'). It found the adware and removed it. Shut down, restarted, ran Spybot again, and nothing. So I'm clean with Spybot, MBAM, SAS and Avast. I'm going to take that as a clean bill of health.