To enable the Hibernate option in our machine, being the best at energy saving, in our Windows 7 equipped computer (and probably also in Windows Vista, although I can't check it right now) we have to type Power options at the Start menu and hit enter.

Once open, we'll see at the left pane a link labelled "Change when the computer sleeps", click on it and it will take us to a new window. There, open the link called "Change advanced power settings", and a third, smaller window will apear:

Deactivate Sleep -> Allow hybrid sleep as seen at the picture above, and, once we accept all the windows, we'll have access to the Hibernate option at the Start menu, wich saves all the information from RAM to the hard disk and shutdowns the computer, allowing for great energy savings and a quick restart next time you power on the PC.