First, the size of the HDD is always and estimate and usually rounded up. You haven't lost any space on it. Second, partitioning is the correct terminology. You still have one HDD, it's just "split" in two for ease of file storage. Files that are best to to move are files that you have placed on the computer such as music, pictures and documents. These files are usually stored under My Documents. Some people store items on their desktop, which isn't actually good for the computer. You should alway store most everything in sub-folders and place shortcuts on your desktop. To start, Right click on the START and click on Explore. This is basically the filing system for your computer. When the window opens it should show the files under the current user (i.e.: Desktop, Favorites, My Documents, etc.) This is all under the C:/ drive. Scroll down and you should see the D:/ drive. Before you move anything, confirm the free space on each drive in the Disc Management window. To get there click on START and right click on My Computer. Click on Manage. In the left screen of this window you should see Disc Management. As long as there is free space, you should be able to move music, pictures and documents from the C:/ drive to the D:/ drive. Just right click on the item you want to move and Cut and Paste. Also, note you shouldn't move files you didn't put there such as the actual My Documents or Desktop folders. Move only the files you put on the computer.
Hope this helps.