You might have seen Security Update 2011-003 in your Software Update. Don’t hesitate to download and install this update.
This update helps quarantine that MacDefender malware that has been making news of late. According to the KB article HT4657KB article HT4657, the update adds the OSX.MacDefender.A definition to the malware check within File Quarantine. Also, the system will now check for malware on a daily basis and “this update will search for and remove known variants of the MacDefender malware. If a known variant was detected and removed, the user will be notified via an alert after the update is installed.”
You can also download the update manually at http://support.apple.com/kb/DL1387