If you've found that your Hibernate option is missing from Windows
Vista, it might be from running the disk cleanup wizard and removing
the hibernate files. This is due to a known bug in Vista that might not
have been hotfixed already.
Turning it back on is very simple, however.
Type cmd into the start menu search box, and use
Ctrl+Shift+Enter to open in administrator mode. You can also
right-click the item in the menu and choose Run as Administrator.
From the command prompt, type in the following command:
powercfg /hibernate on
You'll have to restart your computer before it will take effect.
Note: For more Windows Vista Tips, Tricks & Tweaks.