"Why isnt all the available space on the D drive being used?"
Because, presumably, you've chosen not to use it and loaded any new software onto C.
You may wish to consider un-installing some programs from C and re-installing them on D.
I would advise, however, to leave MS Office where it is (if you have it) but you may have other programs you can re-install onto D.
You also say that you "have cleared memory a few times".
What do you mean by this?
Memory and available hard disk space are two completely different concepts.