Recently while working on a website, I noticed that some words were being underlined. I did not make them links so I scrolled over the words and this popped up:
I did some research and this virus comes from going to download.com (download.cnet.com) and they install it on your computer. There was the day when I trusted download.com, I have long since learned to steer clear of them. They install things on your computer without your permission and then activate it whenever they want.