Whenever you access, copy, save, move, open or close a file, Auto-Protect scans the file to ensure that a threat has not attached itself. By default, it loads when you start your computer to guard against threats and security risks. It also monitors your computer for any activity that might indicate the presence of a threat or security risk. Auto-Protect can determine a file's type even when a threat changes the file's extension.
Please have a look at this KB as well