That happened recently when I was infected with a variant of Virtumonde or Vundo. After visiting dangerous sites computer slowed down and began to display a pop-up window to download the software. You can also get a pop installation that asks Antivirus 2009 which I'm sure most of you are aware now is a deadly virus. I'm not sure if Shield Deluxe developers are paid to run your ads on the virus or virus writers use the name to promote malicious spyware programs.