This sounds like you may possibly have been infected with MACDefender.
The new MACDefender malware was first noted on April 30, 2011 by users of the Apple Support Communities, and was highlighted on May 2 by antivirus company Intego. If the right settings are enabled in Apple's Safari browser, MACDefender can be downloaded to a system after a user clicks a link while searching the Internet.
More details here:
However, users must still agree to install the malware after it downloads. After the ZIP file is extracted, users are presented with the "MACDefender Setup Installer," at which point they must agree to continue and provide an administrator password.
Because of the fact that users must agree to install the software and provide a password, Intego categorized the threat with MACDefender as "low."
Users on Apple's support forums advise killing active processes from the application using the Mac OS X Activity Monitor. MACDefender can then be deleted from the Applications folder by dragging it into the trash. There is also a 'MacDefenderKiller' uninstaller available here:
On May 24 Apple issued this Knowledge Base Article on how to avoid or remove the MacDefender malware (which has been updated several times):
But that does not mean it is over. On May 25, MacGuard is launched:
Further information here:
Apple released Security Update 2011-003 on May 31, 2011, which adds malware detection and removal for the "MAC Defender" scam and delivers a daily update mechanism for updating subsequent malware definitions. This is for Snow Leopard only.
The security update for Mac OS X 10.6.7 is available from Software Update or the company's Downloads page. Installing the update does not require a system reboot.
(The malware is not to be confused with MacDefender, the maker of geocaching software including GCStatistic and DTmatrix. The company noted on its site it is not affiliated with the malware.)
Trojans and other malware spreads through search engines like Google via a method known as "SEO poisoning." The sites are designed to game search engine algorithms and show up when users search for certain topics.
Now also available on Facebook!
Apple are now fighting a running battle with the scareware makers: