I don't know how Sony sets up installations and hidden restore partitions etc. but it sounds like the D drive is a straightforward partition - it just doesn't have any folder structure on it.
You can create folders on it (of whatever names you want) and move files around just as you can within 'My Documents' on the C drive. By default I think all Windows installations set up 'My Documents' within the 'Documents and settings folder on the C drive - you can change this if you want, say to the D drive, it's not too difficult but I think you may need to manually move the existing files to the new location. On the other hand, maybe you just want to keep photos there so just set up folders for each party or whatever.