Microsoft called it a "reimagining" of Windows that will run on all types of devices from small, touch-sensitive screens to traditional large-screen PCs, and that can be used with or without a keyboard and mouse.
It is intended partly to improve Microsoft's position in the emerging tablet PC market, where Windows 7 has struggled to compete effectively with operating systems from Google and Apple. But the new interface is intended for use on all types of PCs.
Microsoft showed several prototype systems on stage running the software, including tablets, laptops and all-in-one PCs. The new interface is a significant departure from the traditional Windows desktop that Microsoft has relied on for decades.
The start screen now has several large, colored application icons that look similar to those on Microsoft's new Windows Phone 7 OS. Tapping an icon with a finger launches the application and allows it to take up the entire screen, without the usual Windows menus, system tray and scroll bars around the edges of the screen.
"The application comes quickly to life as Windows fades to the background," said Michael Angiulo, corporate vice president of Windows planning, hardware and ecosystem, who demonstrated the new software here.
"The tiles on the start screen are live -- they represent your people, your applications, your contacts, the information you care the most about," he said. "You can group them, arrange them and name them as you like, so that first start screen experience is really personal."
The application tiles display new information from the Web automatically, such as Twitter and e-mail messages, or news items from a news reader, he said.
These Web-friendly applications, which Microsoft calls tailored apps, are built on a new Windows 8 development platform and relies heavily on Internet Explorer 10, which will ship with the new OS, and also on HTML 5, Javascript and CSS.
"These languages form the backbone of the Web, so that on day one when we ship Windows 8, hundreds of millions of developers will already know how to build Windows 8 applications," Angiulo said.
The Windows 8 interface can be controlled using gestures on devices that have touch-screens. From within an application, swiping a finger in from the right makes a Windows control panel appear, including a Start button to get back to the welcome screen. From there, new applications can be opened.
Dragging a finger from the left allows the user to toggle between applications that are already open. And dragging a finger up from the bottom of the screen, opens the control menu for each application.
The interfaces shown on several prototype devices here seemed to run smoothly, and Angiulo was able to move quickly through applications and menus.
Windows 8 also introduces a new level of multi-tasking. When a video is playing and the user opens another application, the audio from the video continues to play in the background. A feature called Snap creates a split screen with one application running on each side.
Windows Explorer and the Windows desktop will continue to be available, and Microsoft says the OS will be compatible with current Windows 7 applications and peripherals.
Still, there will likely be much for developers to learn, and Microsoft announced a new conference, called Build, that will take place in Anaheim, California, in September, where developers can learn how to program for the new OS. "Space is limited and registration is open right now," Angiulo said.
Microsoft has also said that Windows 8 -- which it continued to refer to on Thursday as a "code name" -- will run on ARM-based processors from the likes of Qualcomm and Texas Instruments, as well as on the x86 platform from Intel and AMD, as it does today. ARM chips are used in most of today's smartphones and tablets.
Angiulo showed some prototype ARM-based devices running Windows 8, and said ARM processors will not be limited to tablets. "ARM enables ultra-portable computers of all form factors," he said.
Microsoft designed Windows 8 to meet the needs of a changing computer landscape. "The trends we're facing have changed," Angiulo said. "Today we're facing trends around immersive Internet computing, ultra-portable devices, and of course touchscreens. Windows 8 is a reimagining of Windows for those trends."
Several top executives from ARM, Qualcomm and other chip vendors joined Microsoft for its press event. It also previewed the new OS a few hours earlier in the U.S., at the Wall Street Journal's D9 conference.
There was a slight shadow hanging over the event, after a Wall Street Journal report on Wednesday that said Microsoft is requiring the five biggest chip makers to each work exclusively with one PC maker, to help get Windows tablets to market quickly.
Some Taiwanese PC makers have been excluded from the initial program, according to the Journal, and it quoted Acer President Jim Wong saying the PC industry "does not belong to Microsoft, and it does not belong to Intel."
Steve Guggenheimer, who heads Microsoft's OEM division, declined to comment on the report ahead of Thursday's event. Microsoft spokespeople at the event also declined to comment.
However, Microsoft did show a prototype Windows 8 laptop Thursday from Quanta, the giant Taiwanese contract manufacturer, so it clearly is working with some big Taiwanese companies on the new software.
