Remove Mac Malware (Fake Defender etc.)
Apples official removal steps:
http://support.apple.com/kb/ht4650
Start spotlight (using keyboard “command” key + “space bar” key)
type Activity Monitor and press return.
After the Activity monitor opens only “My Processes” list is displayed
and need to be changed to “All Processes” from the drop down menu in the
middle/ upper right corner of the Activity monitor window.
Depending on the malware variant name you are removing you need to find it and select it under the “Process Name” column.
Once selected click the red stop sign “Quit Process” button which is
located on the upper left corner of the Activity Monitor window and
click on “Force Quit”
Open Finder and go to the Applications folder.
Again depending on the malware name locate it in the Applications folder
and drag it to the trash. (make sure to empty the trash once done)
Aditionally you should remove the malware from the Login Item list
which is located in the user accounts under System Preferences.
Using spotlight type accounts and press return.
Click on the Login Items button.
Select the coresponding malware name you are removing and click on the minus button located below the list of Login Items.
Make sure to install all available updates from the Software Updates under the apple menu.
Consider getting antivirus. 
Another good website with removal information: http://blog.macguy.info/2011/05/malware-on-mac.html
