Windows Vista allows you to reclaim some hard drive space by removing the hibernation system file via the Disc Cleanup Wizard. What happens though is that once you do that, the option to hibernate is gone. Its not in the shutdown options, its not in the power options, its just gone.
Here's how you get it back:
Open an elevated CMD prompt (type in "cmd" into the windows search box, right click on "cmd" in search results, and click "Run as administrator")
type in "powercfg -h on" and hit the ENTER key
Voila, you've got Hibernate...