[masraum]

My guess is that it's a pop-up ad from the people wanting you to buy the antivirus.

Virii don't warm you that you're getting infected and prompt you to install stuff to get rid of them. It could also be malware, which does that sort of thing. What is the name of the software that it wants you to buy? If you have malware or something, then you'll need a clue to figure out which one. The clue will be the exact message that you get telling you that you have a problem and the software that it tries to get you to install.

If you can get us those two things, then we can offer a more informed opinion and what, if anything you need to do.

Do you only get that warning when you go to a particular website or perform a particular task?