Could you post a screenshot of Windirstat's readings (arrange the directories by size like on this
screenshot) and another one of the disk management ? To access disk management, right-click "my computer" => manage. Click on disk management and maximize the window so that we can see all the information.
To post a screenshot press the printscreen (or prt scr) key on the keyboard, go to start => all programs => accessories => paint. Click edit => paste then save the image as a .jpg. Use the manage attachments button from the advanced options here to attach the image to your post.