AHHHH!
If you keep getting this pop-up regardless of what site you are on, your computer is infected with malware. If it's Antivirus 360 or Antivirus 2009, then it's a really nasty one to get rid of (and often brings in some vile Trojans.)
First, get a good virus scanner, such as AVG. (I use Avira, both are free.)
If you do indeed have Antivirus 360o r2009, then you will need to get Malware Bytes. I have never successfully removed AV360/2009 without Malware Bytes. Luckily Malware Bytes is free and easy to use.
Right click on the little shield by your clock. If it includes the option to "Exit" do so.
Scan with AVG or Avira.
Scan with Malware Bytes.
Scan again with virus scanner.
MacAfee, Symantec, and Norton are no good against Antivirus 360/2009.