If you have never emptied your Recycle Bin, try that first. Right-click on the Recycle Bin icon on your desktop and select Empty Recycle Bin. Then see how much space you have now. Then, if you need more space (and you probably will), do the copy of the My Documents folder or any other folder that is taking up a lot of space (Audio files and video files are the worst offenders. You probably don't want to move any program files.).

After you have copied the folder to Disk D, you must delete the contents from Disk C. You do this by selecting them and either pressing the delete key, right-clicking and selecting Delete, or dragging them to the Recycle Bin. Then right-click on the Recycle Bin and empty it. You will then have that space back on your C drive.