That "Local Drive D" is unusual. What I see on my Vista system is;

Local Disk (C: )
Volume (D: )
Volume (E: ).

I've renamed both Volume's D and E but that is just window dressing, my way of keeping track of drives.

Vista is installed on "Local Disk (C: ). Can you confirm which drive Vista is installed on your system?

Also, what files did you transfer from D to C?

And finally, using "Computer", (what used to be called "My Computer" on pre-Vista systems), tell us the sizes of these disks and how much free space there is.

Mark