|Here's a good starting point.|
Be sure your Pictures are intact before you delete C:\my Pictures. Same for Documents.
Now you can add My Music to the lot.
Programs are a little different, you'll have to reinstall them and select D:\Program files rather than allowing them to go to C:\ by default.
Some programs require that they be on C:\. You'll have to figure out which ones.
Reduce the Recycle bin to 1 to 2% of disk space.
Reduce System Restore from Default 12% to say, 5%,
You can reduce the PageFile to 1.5 times the physical Ram installed. Moving this to D:\ is not advised. It can be split between C: and D: by leaving a small Pagefile on C: and have a larger one on D:.
By now you should have some breathing room on C:. I would consider using a Partition Manager program to create at least 1 more partition in the 213GB that D:\ occupies. Then use Imaging Software to Create a full Image backup of C: and D:. Then you can adjust the size of C:and/or D: to suit yourself, depending on how much extra room you actually gain on C: after you move what you want moved.
I believe the rule of "thumb" for C: is approx 2GB free at all times. Defrag,I believe, requires 15% available Free Space.
I'm sure you'll get other advice on this subject so I wouldn't jump at the first suggestions made.
There is nothing to learn from someone who already agrees with you.