Microsoft is doing something very interesting these days. By changing the way the end user sees windows so dramatically, they are taking the risk of alienating the very users that have been faithful for so many years. Why are they doing it in such a disruptive way ?
I believe Microsoft, like the rest of us, are seing the end of the PC as the inevitable outcome of the gadget frenzy of the past few years. I think they believe the Apple gadget revolution has made people more inclined to go for different devices for different usages. The tablet browsing is becoming the standard browsing experience at home and while traveling, and the phone browing is becoming the standard for browsing while commuting. What Microsoft is trying to do is to push an extremely simple user experience that includes everything the user has come to expect from a PC into a tablet or a touch screen all-in-one.
Basically, instead of having their business killed by a disruptive change in hardware, their want to accelerate the change and be part of it.
The issue is that the end user, generally, when he goes buying, will look at the devices in a stand, and start comparing the devices. He is not concerned by the vendors (except for the perceived quality of the vendor's products), and is even less concerned by the operating system, except for its familiarity. On the familiarity front, the normal end user will not find Windows 8 familiar (they even would find the iPad and the various android devices more familiar). Most end users are not aware that the tablets they are looking are of different kinds, and have different levels of technology in them.
Basically, if the new user interface impresses, it would gain a good market share of the tablets as well as replacing current desktops and laptops, and as the users start expecting all Windows can do in the devices, it would give Microsoft an edge on all kinds of devices. But if it fails, it would endanger Microsoft's PC market itself.
In short, this could give Microsoft a new begining, with the potential to dominate the new computing markets, but it could also break Microsoft completely...