It's not really a pop up per se... It does pop up, but it looks like an error message or something. It tells you you are infected and asks you to press "OK" (the only choice besides the X at the top) to install their program to get rid of it. I'll try to get a screen shot if it happens again. And it did happen even after I ran a spyware scan with Spybot S&D. So far it hasn't done much but be annoying.