Well it depends what you use the disk for and what type of files are on it.
If your disk contains a lot of downloaded files such as mp3 files, digital pictures, or video files then the obvious thing to do is to back these files up to CD or DVD and then delete them from your hard disk.
Video files are usually very large so if you have any of these then either back them up and delete, or delete them if you no longer need them.
Of course you can also backup and delete other personal files you may have created to make space.
The Windows "Start" menu has an option called "Search". Go in there and select "Files and Folders". Within there you can search for files on your hard disk by SIZE.
Search your hard disk for the largest files on the hard disk and if they are personal files maybe you can back them up and delete them.
If they are Windows files, or you dont know what the file is, DONT delete them as it may stop Windows working. Only delete PERSONAL files.