The Windows Old files are what the system has preserved when a re install has taken place. If you are confident you have all your personal data the yes go ahead and delete these files.
You can move them to D: if you wish, simply drag & drop, holding down shift key to Move rather than Copy.
You could also move your "My Documents" etc folders to D: - Google for a guide to moving folders, it is quite straight forward. This gives a measure of protection to your data should you need to re install windows again.