Finally solved this. I ran Spybot the first time and it locked up, then I downloaded an update and ran it again. It worked the second time. When I rebooted, the pop-up didn't come up again. So it was definitely adware or spyware.

I believe there's a hero in all of us that keeps us honest, gives us strength, makes us noble, and finally allows us to die with pride, even though sometimes we have to be steady and give up the thing we want the most. Even our dreams. -- Aunt May, Spider-Man 2