Here's the deal, Every time you surf the Internet you pick up some sort of potential virus. This is done mainly to keep tabs on your surfing habits.
Most of the time these little viruses are harmless, but unfortunately you do get some assholes who try to be a little bit too clever, and make it virtually impossible to remove their programs from your hard drive.
For instance I recently downloaded some free software to protect my computer from spyware and other threats.
Turns out it was the free software that was the problem because even though I have completely removed the software from my hard-drive.
I still get a pop up every time I start my computer from the same people