Access-based Enumeration
Windows 2003 Service Pack 1 includes a new feature called Access-based Enumeration. When making a file share one can use this feature to specify that it should only list files the user has permission to access. Instead of listing all files and folders and give the user access denied message, when the user tries to access a file/folder without the proper permissions.
Download Access-based Enumeration
More Info MS KB303758