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 now 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.
There will be new apps for the Bing search engine, news and sports, and improvements to the mail and photo applications unveiled previously.
Microsoft said manufacturers and developers were at work on new devices and apps designed to make the most of Windows 8's features.
Do not track
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.
IE 10 will be the first version of the browser with "do not track" turned on by default, meaning users can easily decide not to accept cookies.
Microsoft's Chief Privacy Officer Brendon Lynch  
          announced the move on his blog
.
"In a world where consumers live a large part of their lives online, it is critical that we build trust that their personal information will be treated with respect, and that they will be given a choice to have their information used for unexpected purposes," he wrote.
But the advertising industry has raised concerns.
Microsoft's decision risked "limiting the availability and diversity of internet content and services for consumers," said the Digital Advertising Alliance.
Microsoft has launched the most complete preview yet of its forthcoming Windows 8 operating system.