ahstanford:

Assuming your disk configuration remains as shown in your above link to Disk Management, here is what I would do:

Move everything you care about and want to keep from D to C.

Navigate back to where you took that disk managment screen capture

Highlight and right click the D partition.

Delete it

The space formerly occupied by D should now appear as "unallocated"

Highlight and right click C.

Shrink C to your chosen size. Most people get by with 60 GB or so. If you are a heavy gamer or have 50 plus applications you might want to make that 80 or more. I have 35 installed apps and use 60 GB C partition, of which only 20 is occupied.

The amount by which you shrink C should now be added to the unallocated space.

Right click and highlight the unallocated space and make a new D out of all of it, leaving nothing unallocated.

Move the stuff you moved from D to C in step 1 back to the new D.

Buy a second hard drive and backup up the new D to the new drive. Make images of C and or D if you want with something like Macrium Reflect Free Edition and store the images on the new drive as well.

I haven't done this in a while and don't recall the exact terminology of the choices shown when you right click. The terms may not be "shrink" or "expand", but will be synonyms and should be self-evident on sight. If you have issues, post back here first.