[updated 12th March 2007 21:03]

This particular problem went round the internal discussions groups - having told people how to turn on hibernation and suggested a KB article on it, Armelle O'Neal told me that is has already been written up in KB Article (928897).

SYMPTOMS

After you use the Disk Cleanup Tool in Windows Vista, you may experience the following symptoms:

You cannot see the Hibernate option in Power Options.

When you use the sleep feature, the computer does not recover its settings if power is lost.

CAUSE

This problem occurs when the Disk Cleanup Tool disables the hibernation file. The hibernation file must be enabled to access the hybrid sleep feature and the hibernation feature in Windows Vista.
When the hibernation file is disabled, and the hybrid sleep feature is enabled, a backup of open programs and open files will not be saved to the disk when you use the sleep feature in Windows Vista. Additionally, if the computer loses power while the Windows is in sleep mode, open programs and open files will not be recovered, and any unsaved work will be lost.

RESOLUTION

To resolve this problem in Windows Vista, run the powercfg -h on command at an Administrative command prompt to enable the hibernate feature and the hybrid sleep feature. To run this command, follow these steps:

1. Click Start, click All Programs, and then click Accessories.

2. Right-click Command Prompt, click Run as administrator, type powercfg -h on, and then press ENTER.

 If this fails, it is normally because the command prompt is not actually running at an admin level - click here for my "how to tell"