It sounds like your hard drive is partitioned into two sections (C & D) as your description of D doesn't sound like it's a DVD or CD drive.
If you have anything you can move from C to D it will help - it will still be on your hard drive just the other section (D) which will free up the equivalent space on C which is pretty much full now.
Remember to right click the folders you want to move from C to D and select CUT - not COPY, then right click on D and select paste. Don't try to move things like Windows, it won't like that at all. Also check the folder "Program Files" - and if there is anything in there that you don't use anymore highlight and delete. Then empty recycle bin.
If you are not sure whether to delete something or not, don't delete it - find something else. As Stoop suggests, use D to store videos and photos on, you have a huge amount of free space on your hard drive in D.
Hope that makes sense.