Some hard disks are divided into sections called "partitions". If this is the case with yours then you may have a "C" and a "D" (as magicmerlin says, make sure "D" is not your CD drive).
IF "D" is on your hard disk then your laptop does not just "use" it, you have to TELL it to use it.
So if you copy digital pictures from your camera to hard disk tell it to put them on "D".
If you download mp3 files or video files, then just set the program so its puts them on "D" not "C".
While most times Windows will put things on "C" by default, if you want things on "D" instead then YOU have to specify it.