Vista has changed the way power management works. With its hybrid power management when you put the PC into sleep mode. The PC will remain in low power consumption mode for about 3 hours before it switches automatically to hibernate. Well but I know you might just want to have the hibernate option in your shutdown options. Or you might have had this option but it disappeared.
The hibernate button can disappear after a disk cleanup. After you delete the hibernate file. Anyway there is a quick way to get back the hibernate button. Click on your Vista start menu and immediately type command then right click on the command prompt. Make sure you choose to run as administrator.
Then Key the following command:
powercfg /hibernate on
Now check your shutdown options you should see the hibernate option appear again.