There's a bug in Windows Vista where if the Hibernation files are deleted, then Hibernation is completely disabled. When using Disk Cleanup, the Hibernation files are large so make for a likely thing to delete.
Enabling Hibernation is fairly straight forward, you need to open Command Prompt and type the following then press Enter: powercfg -h on
That should re-enable Vista's hibernate again. Hopefully there will be a hotfix from Microsoft addressing this issue, but Service Pack 1 has been released and the bug remains...
