Open up Windows Explorer (not Internet Explorer), then drag and drop the files from C to D. It's as simple as that.
If you download a lot of music files or if you save a number of word processing documents, then the D drive would be a good place to store them. You can also change the location of the My Documents folder
from the C drive to the D drive.
It's also a good idea to start removing any old programs or files that you don't need so that you can free up some space. Now might be a good time to start doing a little "Spring cleaning" on your system. If you search the forums, there are a number of threads that go into detail on how to free up space on your PC.