Mac Defender pop-up

This is the only pop-up I get so I'm wondering if I have a worm or trojen. Kind of annoying as it dosn't actually pop-up over the webpage I'm viewing, but sits behind it and is there when I close the window.

Any way to get rid of this?

Thanks for the help

I would uninstall the app .

I don't have an app. I guess I should have been more clear. It's a pop-up to purchase Mac Defender....

Mac Defender is a malicious Trojan. Here's what Apple says:

How to avoid or remove Mac Defender malware

Which OS are you running? Apple released an update to handle this malware. It's also very easy to remove.

http://www.mac-forums.com/forums/os-x-operating-system/237651-suspicious-mac-protector-application.html