Snow Leopard has the ability to protect your Mac against malware. This protection will locate and, in some cases, delete malware that shows up on your drive. When new malware attacks appear Apple can automatically update the malware definitions on your drive, without requiring any action on your part. That way, there’s no need to download a new Security Update each time.
To enable this feature turn on the Automatically Update Safe Downloads List option in the General tab of the Security System Preferences pane. Regrettably, it doesn’t always work. In theory, after Apple releases a definitions update, the changes should be pushed to your Mac within the next 24 hours, but in my experience this doesn’t always happen.
You can force an update by unchecking and re-checking the above-mentioned option in the Security System prefs. However, it is not a permanent fix. The next time a definitions update is released, you still may not get it within the 24-hour time frame.
Another way to manually update the definitions is via a Unix application named, appropriately enough, XProtectUpdater.
To run this application via Terminal type:<code>sudo /usr/libexec/ XProtectUpdater</code> press Return, and when prompted, enter your administrator password and press Return again. Your definitions will be updated.