I am really irritated right now. I am new to Mac computers, and I was told that Mac's could not get viruses unless you authorized the install. I was just browsing the web, and then all of a sudden I had a pop up that said "You PC has been running slower than usual. Click here to run a virus scan". I tried just closing it, but it just kept popping up. I finally had to force quit Safari. What can I do to get rid of this??? PLEASE HELP!!!!