Read this page it shows you clearly another way to do the command
are you an administrator user of the pc????????????/
http://itsvista.com/2007/01/itsvist...isable-hibernation-in-vista-via-command-line/
Enable Hibernation in Vista
To enable hibernation, simply click on your Start menu, and type
powercfg.exe /hibernate on in your search box. Then press Ctrl+Shift+Enter,
which will run the command line you typed in as an administrator. If you
have UAC active, it will prompt you to approve the elevation of privileges,
and you'll probably see a quick flash of a command line window appearing and
then disappearing. Your work is done!