There's no single reason why Windows 7 boots faster than its predecessors. Instead of inventing a magic new technology, Microsoft has spent a very long time investigating the boot process, uncovering the bottlenecks and finding ingenious ways to reduce their cumulative impact.
For example, early on in the boot process Windows must load and initialise drivers for every device that it finds. Previously it did this one by one, so if one driver was slow and buggy then it would hold up the boot for everyone.
But under Windows 7, drivers are initialised in parallel, several at the same time, so if one takes a while to start then others can continue to load in the meantime.
This won't have a major effect for everyone. It's not possible for Windows to start all drivers at the same time, and some drivers require others to be initialised before they'll work. If your drivers all perform well now, then loading them in groups will make little difference as they're all competing for the same resources.
But what this change does mean is that installing a poorly written or just very demanding driver won't have a major impact on your boot time, and that's very welcome.
Windows 7 services
It's long been known that Windows initialises unnecessary services by default, and that turning these off can help to improve boot times, save system resources and make things a little more secure. Windows 7 sees Microsoft acknowledge this, too, with useful changes that will help your PC to start more quickly.
The process starts by ditching some services that you don't really need, and bringing others together to make the start-up process more efficient.
So in Windows 7 there's no more 'DFS Replication' or 'SL UI Notification Service', for instance. The Terminal Service services also all disappear. There isn't even a specific ReadyBoost service any more, although the ReadyBoost feature still continues.
Windows 7 also provides 'triggers': new ways of launching services only when you need them. So Apple's Mobile Device service might launch only when you plug in an iPod, rather than every time your PC boots.
This won't happen automatically, though – services will have to be rewritten to take advantage of the new features – so we may not see the full performance advantages for some time.
Even then, third-party developers are sure to continue installing services that you don't really need, so you can expect to continue manually tweaking your service configuration for the foreseeable future.
Everywhere you look in Windows 7 there are boot related changes. Some may only shave off a fraction of a second, but it all adds up. Take the new boot animation, for instance. It's small to cut down on disk I/O, and uses CPU optimisations to improve performance.
The pearl animation in Vista has been dumped, saving more time. Microsoft has cut down on the display transitions, so there's less screen flashing before the log-in screen appears. And Windows 7 no longer tries to synchronise the animation and log-in sound, which means it doesn't have to wait until your soundcard is initialised before the boot can continue.
There are improvements to caching techniques, like pre-fetching, that will improve performance beyond just the boot. And Windows 7 also includes optimisations for the latest hardware. If you're using a solid state drive, for example, then Superfetch, ReadyBoost and pre-fetching will offer little benefit, so they'll all be disabled.
Best of all, Windows 7 extends Vista's diagnostic abilities, allowing the system to record each boot time, detect any problems or hold-ups, identify their most likely cause and perhaps even help with a solution (even if that solution ends up just being something like 'try upgrading this software').
It's this sensible combination of improvements to core Windows features, along with the ability to pick out badly behaved or buggy third-party tools, that should help deliver noticeably faster and more consistent PC start-up times when you upgrade.