The Sourcefire Vulnerability Research Team (VRT) has a great blog on MacDefender, the rogue antimalware trojan currently spreading on Mac systems.
http://vrt-blog.snort.org/2011/05/macde ... iants.html
This malware is known by a variety of names, including "Mac Defender", "MacProtector", "Mac Security", "Apple Security", and "Apple Security Center". The blog is filled with excellent technical details and images, and it also has clear and easy procedures for removing it, which I will repeat here:
1. In Safari under "Preferences", at the bottom of the "General" tab (the first tab), uncheck "Open safe files". This will prevent Safari from starting threats like MacDefender automatically after downloading them.
2. Open up "Activity Monitor" (this is in your Utilities folder within Applications)
3. Find "MacDefender" (or whatever the malware is being called, MacProtector, Mac Security, etc)
4. Highlight it then click "Quit Process" which looks like a big red stop sign at the top right of the Activity Monitor screen.
5. Next, open System Preferences, and go to "Accounts". When it appears click on the "Login Items" button, select the program, and then click the "minus" button to remove it from Login Items.
6. Next, navigate to your Applications folder, find the program, drag it to the trashcan, and then empty the trashcan.
Yes. It's really that simple to remove.