Backup utilities look at the archive attribute to determine whether a particular file has been backed up. The system attribute means the file is considered critical to the heath of the OS, so Windows will protect that file.
Not sure if you are familiar with the command prompt? If you search Windows for CMD it takes you to the command prompt. if you type ATTRIB it lists the file attributes before the file names for all files in that folder. You can change the attribute by "ATTRIB filename.ext +s" to make it a system file for instance or -h to unhide etc