Every version of Windows comes with its own promise of an improvement in boot times, an issue where PC performance still varies widely. For Windows 8, the secret sauce will be a combination of hibernating and traditional booting.
On the Building Windows 8 blog, Microsoft outlines how the approach works in detail, but in rather simplified terms, the idea is this: user sessions (which is most running applications) will be shut down, but the Windows kernel is hibernated (written to disk so it can be speedily reloaded). The kernel is relatively small, so this is a faster process than standard hibernation, which is often a slow process since large amounts of session data get written to disk. The new approach also makes better use of multiple processor cores to speed up the process of reloading the hibernated data.
In the video above, the Windows machine boots impressively fast. We'll need a lot more real-world evidence to see how much actual difference this approach makes, and what happens when a system patch demands a "full" reboot rather than this model. I'm also curious about what happens when you reboot after a system crash -- is an older hibernation file used, or do you start again from scratch?
Delivering fast boot times on Windows 8 [Building Windows 8]