As default Windows creates a file (c:\hiberfil.sys) to save the complete memory if you hibernate. It is the same size as your memory e.g.. if you have 2GB Ram, windows will create a 2GB file!. If you don't use hibernate (and who does?) you can remove the file by doing the following:

1. Go to Settings -> Control Panel -> Power Options.
    
2. Click Hibernate tab and uncheck "Enable Hibernation".
   
3. Click Apply and you have gained some Hard Disk space.