A recent scam has been tricking Mac users to install a virus by redirecting them from legitimate websites to fake websites which tell them that their computer is infected with a virus. The user is then offered Mac Defender or Mac Protector “anti-virus” software to solve the issue.

This is not anti-virus software but is indeed a virus itself.

If you downloaded this virus and installed it, the following pages can help you get rid of it:

http://support.apple.com/kb/ht4650?locale=en_US (Manual Removal)
http://www.bleepingcomputer.com/virus-removal/remove-mac-defender (Automated Removal Tool)