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. But Microsoft has found itself sidelined in the rush toward mobile computing by Apple, Google Inc and Amazon.com Inc.

"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.

Anyone can download the new version of Windows, but it will only work on PCs and laptops running standard Intel Corp x86 chips. Consumers will have to wait longer to try out the full experience on ARM-compatible tablets.

It 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.

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.

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 apps and get access to cloud storage with the ability to move content across a range of devices including Windows phones. It also uses a test version of Microsoft's newest browser, Internet Explorer 10.

The Windows Store has a range of apps including a reader for Gannett Co Inc's national newspaper USA Today, games such as Cut the Rope, and other titles compatible with Microsoft's Xbox video game system. But it is still tiny in comparison with the 140,000 apps available for the iPad. Users need to sign on with a Microsoft account to download apps, which are all free during the test period.

The test version of Windows 8 has mail, calendar and messaging apps, but no evidence of Office, Microsoft's dominant suite of work applications. Sinofsky said earlier this month that a version of Office would be included in Windows 8 for ARM tablets.

Microsoft's shares fluctuated on the Nasdaq, but briefly hit a new four-year high of $32.00 in early trading. The shares were down 17 cents at $31.70 in afternoon trading.

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

• A Permanent Economic Emergency