Microsoft presented itself an enormous task when it started development of Windows 8: Produce an operating system that could go toe-to-toe with iOS and Android in the touch-driven tablet space while still preserving the value of Windows' considerable history on the desktop.

This complex problem has challenged Microsoft for about two decades. For 20 years the company tried to ship touch-driven (generally stylus-driven) tablets that used the conventional Windows front-end, hoping a smattering of extra utilities—an on-screen keyboard that floated over the Windows UI, boxes to contain handwriting—would be enough to convert a mouse-driven operating system into a tablet platform.

These tablet machines never achieved substantial success. Apple's iOS, first released in 2007, demonstrated that touch machines could find enormous success as long as their interface was sympathetic to both the constraints imposed by touch—imprecision, obstruction of the view—and its novel capabilities. Before Windows 7 was finished (and before Apple had even announced plans to produce an iOS-running tablet) Microsoft started work on developing such a user interface for Windows.

The result of that work is Windows 8. Over the last few days we've given the Consumer Preview a thorough examination. The operating system has plenty of new features and improvements that we'll cover in the coming weeks and months. But as interesting and important as these changes are, they are small fry until one fundamental question has been answered: Has Microsoft truly succeeded in squaring the circle? Does Windows 8 genuinely have an interface that is equally effective for users of traditional input devices and touch machines? Is Windows 8 an operating system for tablets and desktops alike?

The tablet as a PC

Microsoft didn't deviate from the position it asserted over those many years of poor tablet sales. For Redmond, the tablet is not a category apart form the PC. Tablets are "a sort of PC," albeit one usually lacking a mouse and keyboard, gaining a touch screen in their stead. The new user interface it had to develop for Windows would respect this position. Built to enable touch-screen tablet computing, but also effective with traditional PC input devices. The goal was to produce an interface that was all things to all people.

To cater to the needs of touch users, Microsoft went back to the drawing board. Those stalwarts of traditional user interfaces, Windows, Icons, Menus, and Pointers, have been left behind in favor of a new design concept called Metro. The Metro aesthetic calls for simple shapes, crisp and high quality text, then careful use of juxtaposition and layout to convey information. Ultimately: visual simplification.

Traditional interfaces are chock full of artifice—3D-effect buttons drawn to look as if they "stand out" from the background and get "pushed in" when you click them, inlaid panels, chunky window buttons, and more. Metro discards these. They're all essentially unnecessary. We computer-users are experienced enough to know an icon or a piece of text that's colored differently from the surrounding text is probably interactive. It will do something when we tap or click it. The ornamentation isn't conveying useful information any more.

Armed with the Metro idea, Microsoft built a new shell for Windows codenamed "Modern Shell" or "MoSh." Along with the new Metro shell are new Metro applications, both first- and third-party, so that applications and operating system alike are built for touch.

Modern Shell basics

The centerpiece of the Windows 8 is the Start screen. This is a big, bold, brightly-colored full-screen application launcher/system dashboard, packed with neatly organized tiles. Unlike the icons in the Start menu, the Start menu's code has been at least partially removed from the operating system. There will be no simple registry hacks to reinstate it—the tiles in the Start screen are living things. These live tiles keep themselves up-to-date. The Mail tile does not simply start the mail program, it shows how many unread mails you have. The Weather tile shows the current conditions.

Tapping any one of those tiles launches the corresponding application.

New-style Metro applications, like the Start screen, are also full-screen. For all Metro's aesthetic differences, it's the windows—specifically, the lack of windows—that are most striking. The resizable, overlapping, movable boxes with their title bars and borders and menus that have defined the historic Windows user interface—all gone. Metro apps, like the iOS apps Apple has been so successful with, are full-screen, edge-to-edge affairs. Every single pixel is dedicated to the app. When an app is running, the operating system doesn't merely take a back seat: it disappears entirely.

On systems with a respectable resolution (1366×768 or higher), in addition to the full-screen view there is a split-screen mode. The split is fixed and asymmetric. Two applications can be shown at one time. One takes up the majority of the screen, the other is constrained to a narrow ("snapped") sidebar.

As well as being full-screen, apps also work a little differently than they used to. Instead of conventional toolbars and context menus, every application can provide an "app bar." It's a strip of commands along the bottom of the screen. These remain hidden until called into view with a swipe or a click. Application configuration is also handled in a new way; it's performed from the charms bar.

The charms bar is an always-accessible set of five icons that appear down the right hand edge of the screen. From top to bottom, these icons are Search, Share, Start, Devices, and Settings. Start is the easiest to handle as it simply takes you to the Start screen. Search is used to initiate searches, both of built-in things (files, programs, settings) and applications that register themselves as being searchable. Share allows the content of the current application to be shared in some way with other applications (for example, a Flickr browser might allow URLs to be "shared" with other programs, and a mail client might opt to receive such shared content).

Devices allows certain configuration of and connection to other devices. For example, a DLNA networked music player might be listed in the Devices charm, allowing a music application to easily send music over the network to the device. Settings contains configuration for both the system as a whole and the currently running application.

The final interface concept new to Windows 8 is "semantic zoom." Applications that present long, grouped lists (including the Start screen itself) allow the user to zoom out to gain a bird's eye view of the list. This zoom doesn't simply make the list items smaller; they actually change to become simpler and make the group names more prominent. For example, semantic zooming the Start screen replaces the live tiles and text with simple icons for each tile.