Microsoft is launching the most complete preview yet of its forthcoming Windows 8 operating system.
The company said that tens of thousands of improvements have been made to what it calls the most important redesign of its interface since Windows 95.
Consumers can download the release preview of Windows 8, a system which Microsoft says is its most tested operating system ever.
It is expected to go on sale in the autumn, three years after Windows 7.
The new operating system is designed to bring Windows into the touchscreen, smartphone era.
It adopts the Metro interface of the company's mobile operating system, Windows Phone 7.
Windows, which still dominates the desktop PC software market, has been much slower to make an impact on mobile phones and tablet computers.
The release preview has features not available in the last version, the consumer preview, launched in February.
The latest version of Microsoft's browser Internet Explorer 10, optimised for touchscreen, is also included for the first time. Users are promised greater personalisation of the start screen, and more control over privacy.There will be new apps for the Bing search engine, news and sports, and improvements to the mail and photo applications unveiled previously.
Internet Explorer 10 will be the first browser with "do not track" turned on by default, meaning users can easily decide not to accept cookies.
Microsoft said manufacturers and developers are at work on new devices and apps designed to make the most of Windows 8's features.