If you have a network backup you dont need to have it all on the C Drive. Just download what program off the network and usei it on C drive and when you are done save it all back to the Network.
C - OS and a few applications you care for them to perform more
D - remember that “D” is for Data, all of that goes here and less performance minded applications go here (to be honest you may not even notice performance differences but maybe a boot up speed increase C versus D).