Move iTunes data from C drive to D drive
(1)In my system Itunes music folder can't be moved "from Windows", but I can move the Music folder containing it (Right-Click, Properties, Location).  (It can save some space on C, but is not the itunes folder with the backups).  The first step to free up space on C should be to move the folders that can be moved this way, like Videos, Photos, Documents.

(2)On another Windows machine I have used junction points to have the directory for backups physically on another drive.  (While still the same path from the point of view of Itunes).  

If it is the same physical drive, I would reparation before doing (2).  You can also move swap, temp, hibernate files etc.