If you discover a trojan program is running on your computer then look to the following information for assistance:
1. A recent discussion on the Apple Support Communities: MacDefender Trojan.
2. An excellent site devoted to Mac Malware: Macintosh Virus Guide
3. Another site for removing MacDefende, et.al.: MAC Defender Rogue Anti-Virus analysis and Removal
4. A new removal utility - MacDefenderKiller
5. And to protect against a recent variant, MacGuard.
Before you delete anything, we need your help. Some AV folks in our community need to analyze these files in order to protect others. Before you delete anything please consider doing the following: Upload either the original .zip file or the MacGuard application to http://www.VirusTotal.com. If either is not detected by ClamXAV, then also upload it to http://cgi.clamav.net/sendvirus.cgi. If you are uncomfortable doing this for any reason and can determine the URL of the site where you got it please send the link to firstname.lastname@example.org.
Removing strange software can be a task. The following outlines various ways of uninstalling software:
Uninstalling Software: The Basics
Most OS X applications are completely self-contained "packages" that can be uninstalled by simply dragging the application to the Trash. Applications may create preference files that are stored in the /Home/Library/Preferences/ folder. Although they do nothing once you delete the associated application, they do take up some disk space. If you want you can look for them in the above location and delete them, too.
Some applications may install an uninstaller program that can be used to remove the application. In some cases the uninstaller may be part of the application's installer, and is invoked by clicking on a Customize button that will appear during the install process.
Some applications may install components in the /Home/Library/Applications Support/ folder. You can also check there to see if the application has created a folder. You can also delete the folder that's in the Applications Support folder. Again, they don't do anything but take up disk space once the application is trashed.
Some applications may install a Startup item or a Log In item. Startup items are usually installed in the /Library/StartupItems/ folder and less often in the /Home/Library/StartupItems/ folder. Log In Items are set in the Accounts preferences. Open System Preferences, click on the Accounts icon, then click on the LogIn Items tab. Locate the item in the list for the application you want to remove and click on the Delete [-] button to delete it from the list.
Some software use startup daemons or agents that are a new feature of the OS. Look for them in /Library/LaunchAgents/ and /Library/LaunchDaemons/ or in /Home/Library/LaunchAgents/.
If an application installs any other files the best way to track them down is to do a Finder search using the application name or the developer name as the search term. Unfortunately Spotlight will not look in certain folders by default. You can modify Spotlight's behavior or use a third-party search utility, Easy Find, instead. Download Easy Find at VersionTracker or MacUpdate.
Some applications install a receipt in the /Library/Receipts/ folder. Usually with the same name as the program or the developer. The item generally has a ".pkg" extension. Be sure you also delete this item as some programs use it to determine if it's already installed.
There are many utilities that can uninstall applications. Note that you must have this software installed before you install software you may need to uninstall. Uninstallers won't work if you install them after the fact. Here is a selection:
For more information visit The XLab FAQs and read the FAQs on removing software and dealing with spyware and malware.
After removing all the components of the software you may have to restart the computer to fully disable the software. This will be the case when removing software that has installed a daemon. After the daemon has been removed you need to restart the computer to stop the daemon. Alternatively, you can kill the daemon process using the Terminal application or Activity Monitor.
MacKeeper is not the same as MacDefender, so the instructions specific to MacDefender that Kappy posted won't work for MacKeeper. It has also proven to be quite stubborn and difficult to get rid of, so you should follow the directions here:
That should get rid of all the MacKeeper evilness!