Assuming all your files are in your User Profile, move that to the D partition.
Right Click on each folder listed below, click on Location Tab and change the location from C to D. Don't change anything except the drive letter. New folder will be created and the data moved. Unfortunately you need to do this for each folder:
Contacts
Desktop
Downloads
Favorites
Links
My Documents
My Music
My Pictures
My Videos
Saved Games
Searches
The good thing is once that is done, if you ever need to reinstall the O/S onto the C partition then all your data will not be over written. You would be advises to back it up before hand though and you will need to repoint to the User lacation on the D partition.
In case it isn't mentioned (I may have missed it), clean out your Temp folder.
C: \Users\
UserName\AppData\Local\Temp (system files and folders don't get moved as per the above User folders, just data.
CCleaner can also be used but be advised it does remove items you may use everyday like saved logon cookies etc.