NIS does not appear in right-click menu for some system files

I would like to scan my hiberfil.sys file because Avast said it was infected before I removed it and installed NIS 2012.  I wanted NIS 2012 to scan it, but when I right click on it or many other system files, NIS does not appear in the right click menu.  I was able to create a custom scan to scan the file, but I am curious why this behavior exists.

 

Anyone know?