Windows 8, as the first Microsoft operating system compatible with low-power microprocessors designed by ARM Holdings Plc, will run on tablets as well as desktops and laptops, in an effort to counter the runaway success of Apple Inc's iPad.

"It's an even better Windows than Windows 7," said Steven Sinofsky, head of Microsoft's flagship Windows unit, as he demonstrated the new system at the Mobile World Congress in Barcelona. Windows 7, Microsoft's last operating system, was its fastest-selling ever, racking up 525 million sales in less than three years.

"It's incredibly fast and fluid to just navigate this UI (user interface)", said Sinofsky, showing off Windows 8 on a tablet and an ultra-thin laptop at the event in Barcelona.

The test version of Windows 8 is available for download at preview.windows.com.

Sinofsky said people from 70 countries had already downloaded the software on Wednesday morning, but he did not give exact numbers.

The downloadable version will only work on laptops and PCs running traditional Intel x86 chips. The ARM-compatible version of the system is not yet ready for public use. Microsoft has said it will distribute ARM tablets running Windows 8 to select developers at some point.

Microsoft says it is aiming to get machines running on both the ARM and Intel platforms into the market at the same time but has not set a target date. The world's largest software company generally tries to bring out a new version of Windows every three years, so that would indicate a full release date around October this year, in time for the holiday shopping season.

In both versions, Windows 8 features a completely new interface, borrowed from what Microsoft calls the "Metro" style of the current Windows Phone software. It features blocks or "tiles" that can be moved around the screen or tapped to go straight into an application.

Microsoft said the latest version of Windows 8 unveiled on Wednesday has better performance, quality and reliability than the version it released to developers last autumn.

For the first time, this version of Windows 8 includes the Windows Store, where users can download and try out free apps, get access to cloud storage with the ability to move content across a range of devices including Windows phones, and download a test version of Microsoft's newest browser, Internet Explorer 10.

Microsoft's shares fluctuated on the Nasdaq, but briefly hit a new four-year high of $32 in early trading. The shares were down 8 cents at $31.79 near midday.

(Reporting By Georgina Prodhan in Barcelona and Bill Rigby in Seattle; editing by Matthew Lewis)

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