Are you sure that the D isn't just a small partition used to hold information for a reinstall? In My Computer, right-click the D: drive and go to Properties. If it's only 1 or 2 Gb in size, I'd say it's purely for that purpose.
Otherwise, you could use the whole D: as "My Documents"... Click on Start, then right-click on "My Documents". Go to Properties and in "Target", type "D:\". All the "My Documents" files on the C:\ can then be copied to the D:\ and when you save to My Documents, it will automatically go to the D: drive.
Also, you should never fill the C: drive close to its capacity as this can impair the functionality of windows...