You can move your My Documents folder to the D:\ partition quite easily.
First create a folder on your D:\ drive titled My Documents. Now open windows explorer, right-click My Documents and select properties. You should get a window like in the attached screen shot. Click the "Move" button and navigate to the new folder you created on the D:\ drive, select it and OK. This will move the My Documents folder to the D:\ drive and free up some space on your C:\ drive. This process is invisible to the user. Nothing will change as far as accessing My Documents folder. The effect will be dependent on just how much data is in your My Docs folder.
Alternatively, you can resize both partitions so C:\ is larger and D:\ is correspondingly smaller. This is a pretty simple task with a free download program called
Easeus Partition Master. I've done that very operation on a couple of systems using that program.
Raybro