I'm assuming it's an Ultimate thing... I never used to reboot before, just hibernate
Hibernate is available on all Windows Vista versions (well, maybe not Starter), however if your computer supports "Hybrid Sleep" it will not be available.
With hybrid sleep, using the sleep option on the start menu saves state to the hard disk (like hibernate) but doesn't turn off power (like standby). If your system loses power while in hybrid sleep, it will still be able to resume as if it was resuming from hibernation. Additionally, after a configurable period of time (the default is 2 hours if on battery and 16 hours if on AC) the system will hibernate properly automatically.
Hybrid sleep works fantastic imho. My laptop supports it (my desktop does not), I always use hybrid sleep so if I return to it within two hours it resumes within a second, and if its longer its hibernated so I don't waste battery, and can still resume in about 15 seconds.
You can disable hybrid sleep in the power options (choose "Change plan settings" for the active power plan, then "Change advanced power settings", then it's under "Sleep", "Allow hybrid sleep"; if this option is missing your computer does not support hybrid sleep), after which the regular hibernate option returns to the start menu.