1. Right-click on Start and select Explore to get into Windows Explorer.
2. In the left section, click on My Computer. (On the right, you will see all your drives. )
3. Click on the + next to My Computer to expand it. Then go to the files you want to transfer.
4. Select one or several of those files or folders, right-click and select Cut.
5. Now click on your D drive in the left section and go to where you want to put the files.
6. Right-click in the right section of Windows Explorer and select Paste.
You should browse your folders and do a "alt-enter" on them to see which folders take up the most space. Certain folders you should probably NOT move, ie anything under "Program Files". Files under "My Documents" are usually safe, although music files like those under the iTunes folder may cause problems if moved. Basically, if you created the folder, it's safe to move it. If a program created it, open the program up, look at options, and see if there's an easy way to change location of that folder