You could make "D" into two partitions 10 GB and 50GB . Put all the contents pof "D" into the 50 GB . Move as much stuff rom "C" to the new "D" . Merge the 10 GB with"C" creating a new "C" 28 GB .
Your OS needs to be in the first partition "C" as a general rule . Playing around with partitions is fairly straight forward but if you get it wrong you can loose all your stuff and end up having to do a clean install . It pays to read up on the program you will use and double check every move . A pen and paper comes in very handy .