How to avoid or remove Mac Defender malware

Go to the Utilities folder in the Applications folder and launch Activity Monitor

Choose All Processes from the pop up menu in the upper right corner of the window

Under the Process Name column, look for the name of the app and click to select it; common app names include: MacDefender, MacSecurity or MacProtector

Click the Quit Process button in the upper left corner of the window and select Quit

Quit Activity Monitor application

Open the Applications folder

Locate the app ex. MacDefender, MacSecurity, MacProtector or other name

Drag to Trash, and empty Trash

Malware also installs a login item in your account in System Preferences. Removal of the login item is not necessary, but you can remove it by following the steps below.

Open System Preferences, select Accounts, then Login Items

Select the name of the app you removed in the steps above ex. MacDefender, MacSecurity, MacProtector

Click the minus button

via How to avoid or remove Mac Defender malware.