Do the following:
1. Click Start then click Run. Then type compmgmt.msc in the Open box and then click Enter.
2. In the left pane, click Disk Management.
3. Right-click the Partition or Volume that you want to mount, and then click Change Drive Letter and Paths.
4. Click Add.
5. Click Mount in the following empty NTFS folder (if it is not already selected), and then use one of the following steps:
Type the path to an empty folder on an NTFS volume, and then click OK.
Click Browse then locate the empty NTFS folder, click OK, and then click OK.
If you have not yet created an empty folder, click Browse, click New Folder to create an empty folder on an NTFS volume, type a name for the new folder, click OK, and then click OK.
Quit the Disk Management snap-in.
Of this sounds to complicated then all you have to do is to make sure the D drive has a partition (Which it already has) and it is formatted (Which it is) and then just move files from the C to the D drive.
When you want to save files save them to the D drive then. If you run out of room on your C drive the system will probably cease to function correctly before this happens.