I might suggest the opposite, depending on some circumstances. For example, do you have an option of burning recovery discs, such as the utlity HP includes on their systems? If so, do this, and then you can delete D, shrink C, and then make a new D partition to be the size you want.
Having your only recovery option stored on your hard drive is never a good idea. If the drive goes, not only is your data gone, but so is your recovery. With the discs, you could replace the drive, and be back up and running shortly thereafter.