The D: drive you are talking about is most likely the Recovery partition installed by the computer manufacturer. It's your way to restore the system to it's factory installed state. Not a good idea to delete it. If you do decide to delete it, please make sure you create the set of recovery DVDs. It should be an option provided by the computer manufacturer from the Start Menu, All Programs.
As for saving to C: instead of D;, where are you being prompted to do this from? Is it when you download a files from the internet, do a Save from a program, ???? Need more info. There is no universal place you can change that I'm aware of. Normally, once you tell a program to save to a folder on your C: drive, the next time you save from that program, it will point to the last place you saved to.