Security software firm Intego has identified a new piece of malware that masquerades as an antivirus program called MACDefender. This is in no way associated with the German company called MacDefender that runs the MacDefender.org website and writes geocaching and other GPS-related software.
In order for you computer to be infected by MACDefender, you must visit a poisoned website, allow the web browser to open files after the file downloads itself, and then enter an admin password in the installer. MACDefender then adds itself to the login items, displays a menu bar icon, and appears to be a legitimate antivirus program. The goal of the malware, however, is to scam users into paying for the program and possibly steal credit card numbers as well. To that end, it pops up notifications every few minutes that a virus has been found in an attempt to make the user think they're infected.