It depends what it is full off.
By far the largest files on a hard disk are usually video files or movie files.
Have you got any video files that you could perhaps back up to CD or DVD, or if you dont want them delete them.
Windows has a Search option which allows you to search for files BY SIZE. So you could do a search and find the largest files on your hard disk.
Once you get a list of the largest files you can decide if you want them or not, and if not delete them.
Remember some may be Windows system files so dont just delete something without knowing what it is.