A lack of apps is Microsoft's Achilles heel as it attempts to catch Apple Inc and Google Inc in the rush toward mobile computing.

Windows 8, the new Surface tablet and a range of Windows-based phones - all unveiled in the past week - are designed to close that gap, but the world's largest software company still needs to convince developers to recreate the thriving 'ecosystem' that made PCs so successful.

"Please go out and write lots of applications," Microsoft Chief Executive Steve Ballmer pleaded with 2,000 developers on Tuesday, kicking off an annual, four-day meeting at its campus near Seattle.

The event, called 'Build,' is the equivalent of Apple's Worldwide Developer Conference and Google's I/O event.

Microsoft gave each paying attendee one of its Surface tablets and 100 gigabytes of free space on its SkyDrive online storage service. On top of that, handset partner Nokia threw in a free Lumia 920 smartphone running Windows Phone 8.

The unprecedentedly generous give-away signals the intent of what Microsoft openly calls "evangelism." Most developers at the meeting, who paid up to $2,000 to attend, are already converted to the Windows religion. But this year there is a feeling that Microsoft can re-establish itself as a relevant platform for developers.

"The sessions are overflowing. Everybody wants to learn," said Greg Lutz, product manager at development tools company ComponentOne, who is attending the conference.

"The Surface is really exciting. It's been interesting to see people that would normally be critics of Microsoft surprised to see how good it is," said Lutz, whose company makes features that developers can use in apps, such as calendars or charts.

Microsoft recognizes it needs apps to flesh out its new online Windows Store and make Windows 8 machines more attractive to users, said Russ Whitman, chief strategy officer at Ratio Interactive, a design agency that helps companies create apps.

"The catalog (of apps) is where they are weak, there's no doubt," he said. "But if Microsoft stays focused on quality not quantity, they can win."

DEVELOPER DOUBTS

When Windows 8 launched on Friday, some major content providers had prominent apps in the Windows store, such as Netflix Inc, the New York Times and Rovio's Angry Birds Space. But big names such as Facebook and Twitter were missing.

Twitter moved to rectify that on Tuesday, announcing that a native Windows app would be rolled out "in the months ahead." Dropbox, a fast-growing cloud storage service, also announced it would soon have a Windows app, as did online payment firm PayPal and sports network ESPN.

But Facebook, which now has more than 1 billion users, has not yet made public any plans for a Windows app, despite the fact Microsoft is a minor shareholder.

And Microsoft still has to overcome indifference from many developers who do not see demand from users or simply do not have the resources to build Windows apps alongside iOS and Android.

"Windows 8 is getting good reviews and the tile user interface is a great fit with our geo-visual content," said Jason Karas, CEO at website Trover, where users can share photos of interesting discoveries. "It's on the roadmap for Trover, but we are still a very lean team, so we're hesitant to support a third platform until we have all the innovations we want to see in iPhone and Android in place."

Microsoft has yet to persuade other influential online services, for example car-rental firm Zipcar or real estate information firm Zillow, to develop for Windows 8.

To get more developers on board, Microsoft is spending this week demonstrating how it is making it easier to develop apps for Windows and get them into the real world.

A key part of that is a new set of tools tying in its Azure cloud service, which allows Windows apps to easily harness data stored in remote servers.

"Some of the new changes are pretty incredible and are going to make developing, especially some of the mobile apps, much easier," said Mike Cousins, a software developer following the conference by webcast from Calgary, Canada.

"It just makes it super-easy to integrate mobile clients into your application," said Cousins, who is developing Shuttr, a site for photographers to display and sell their work. "It's been reduced from probably a week's work to minutes."

400 MILLION NEW MACHINES

Microsoft's best argument to developers is the sheer size of the Windows user base.

Microsoft sold 4 million upgrades to Windows 8 in its first four days, a mere fraction of the 670 million or so machines running Windows 7. Ballmer said there would be 400 million new devices running Windows next year, including PCs, tablets and phones, and the company would be marketing heavily to consumers.

That is an attractive audience for developers, and Whitman at Ratio Interactive said he saw many new faces at Microsoft's event this week who previously were more interested in web-based apps and other platforms.

"There's a new generation of developers that can build on Windows 8 that have been building using JavaScript and HTML," he said. "Seeing some of those developers show up and talk about building apps using other languages is pretty cool. It's a whole different group than Microsoft has traditionally been able to court."

One Wall Street analyst said developers may even be tempted to switch back to Microsoft after working with Apple's iOS platform.

"There does seem to be some excitement about the new operating system and many of the new devices that are coming to market," said Jason Maynard, an analyst at Wells Fargo Securities. "We have heard some developers talk about 're-Microsofting' and moving from their Macs for app development."

Cousins said that once developers see the user base for Windows 8 grow, the momentum will start to have an effect.

"All the new PCs people buy will be Windows 8, and people will start demanding Windows 8 apps from companies, and then they will start making them," he said. "I think we'll see a wave of apps coming out pretty soon."

(Reporting By Bill Rigby; Editing by Steve Orlofsky)

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