We're going to need some more information.
Most computers call the hard driver 'C'. The letter 'D' is often used to refer to optical media drives (such as CD or DVD drives). I'm guessing that you've either got two hard drives ('c' and 'D') or that you've got one hard drive partitioned so that it appears as two separate drives. But it would be useful to know what you're actually referring to when you write about your 'D drive'.
Assuming that you have got 2 hard drives (or 2 partitions on a single drive), the most common way of organising them is to have all of the system files (i.e. Windows) and program files (e.g. Microsoft Word) on drive C, with documents, images, videos etc all stored on drive D. If so, then as long as you've backed up all of the files on dive D, you can delete everything that's on it if you want to (and if you're really sure that there are no system files or program files on it).
In short, you'll never damage your computer by deleting things like documents, images, videos, spreadsheets, databases, etc but you should never delete system files or program files (other than by the proper process for uninstalling them of course). You know what's on 'D'; we don't. Without further information from you, you must decide for yourself.
Chris