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 eg. 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 and uncheck 'Enable Hibernation'

3. Click Apply and you have gained some Hard Disk Space