First check how big your hard drive is (right click on the icon in My Computer and click Properties).
Then check it is not split into 2 or more partitions. Most people have just a "C" partition that takes up all the disk but it is possible to have a "C" and a "D".
If you do have two partitons maybe C is full up and D is empty.
Have you been downloading movies, or songs, or putting loads of digital pictures on your hard disk?
These can all take up lots of room, particularly movies and video.
If you have lots of personal files maybe you need to back some up onto CD or DVD and delete them from your hard disk. This will free up space.
If you look on the Start menu there is an option Search. Select this then "For Files and Folders".
A search window opens, select "All files and folders". Now you can search for files by size. Start with a large size, and if you keep getting nothing reduce the files size a bit at a time until you find something.
Maybe you have some large video files you forgot about. Maybe you can back them up or delete them.
IF YOU FIND A FILE YOU DONT RECOGNIZE DONT DELETE IT, IT MAY BE AN IMPORTANT WINDOWS FILE.
If you need more help ask again.