SAN FRANCISCO (Reuters) - Microsoft Corp released a test version of its Windows 8.1 software on Wednesday, bringing back the 'start' button and adding a host of features it hopes will appeal to users, while spurring developers into writing more applications for it.

The updated Windows, which was signaled at the end of May, is aimed chiefly at soothing traditional computer users, many of whom were unsettled by Microsoft's shift towards a new 'tile'-based interface that works best on touch-enabled devices, but left fans of the old-style desktop confused.

"Since we announced and shipped Windows 8, suffice it to say, we pushed boldly and yet what we found was we got a lot of feedback from users of those millions of desktop applications," said Microsoft Chief Executive Steve Ballmer, opening the company's annual developer conference in San Francisco.

"If I was to put it in coffee terms, 'Why don't you go and refine the blend here?'. Let's remix the desktop and your modern application experience. Let's balance them better," said Ballmer, summing up user feedback.

The result is the reinstatement of the 'start' button, and easier ways to find and access applications, along with a highly improved search function, some of which was announced last month.

Ballmer also promised a "rapid release cycle" for Windows in future, abandoning its previous policy of making new versions of Windows every three years, in an effort to match Apple Inc and Google Inc.

The response from the thousands of developers at the conference in San Francisco was broadly positive, although attendees tend to be Windows die-hards.

"Of course, they're playing some catch up (with Apple and Google). They have been lagging behind for years now," said Jorgen Nilsson, manager at UK-based Aveva AB, a firm that makes computer-aided design software applications. "But this release is driving it forward instead of catching up and making it work for business and personal use. This is looking really good now."

Part of Microsoft's problem has been persuading developers to create apps for Windows 8 and the little-used Windows Phone, given that almost all smartphone and tablet users are accessible with Apple's iOS or Google's Android system.

Microsoft made a big leap on Wednesday by announcing that Facebook Inc has finally agreed to work on an app especially for Windows, which should be available this autumn. That is one factor that might help attract the more than 1 billion Facebook users to Windows-based tablets.

"I feel like Microsoft can actually seriously compete in the mobile ecosystem now," said Manav Mishra, director of engineering at the Barnes & Noble Inc unit that makes apps for its Nook e-reader. "Windows 8.1 finishes the journey Windows 8 started and I think it evens the playing field for Microsoft quite a bit, which wasn't the case before."

Microsoft shares were up 2.2 percent at $34.39 on Nasdaq.

(Reporting by Malathi Nayak in San Francisco, writing by Bill Rigby in Seattle; Editing by L Gevirtz)

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