At the simplest level, with iTunes closed copy the entire iTunes folder from <User's Music> on your C: drive to D:\ as D:\iTunes then hold down shift as you start iTunes, click choose and choose the library database file at D:\iTunes\iTunes Library.itl.
This will normally work fine, but if your library started life prior to iTunes 9 there might be some optimization to do before you copy it. If the media folder is inside the iTunes folder called iTunes Media and contains a Music folder with the artists and albums it will be fine. If your media folder is called iTunes Music then it was made with an older version of iTunes. It can be updated to the latest layout with the method in my post on making a split library portable which should also be used if the media folder is in a non standard location.