Within a year, the newly launched Windows 8 will likely be powering almost 400 million PCs andtablet computers.

Microsoft has built an application marketplace, the Windows Store, right into its new operating system. It's a place for consumers to find software programs to make their new machines more productive, more useful, and more fun.

But even though Microsoft announced the Windows Store more than a year ago to developers, the marketplace has about 5,000 applications for users in the United States, a few thousand more globally. If the opportunity is so big, why are the offerings so scant?

"They need to make themselves relevant," said Forrester Research analyst John Rymer.

That seems like an odd task for a company that has millions of developers writing programs for its various platforms. But the business of software development is changing. Increasingly, computer users are handling the tasks that were once the domain of packaged software, where Microsoft has had plenty of success, with lightweight, specialized mobile apps and Web services, where Microsoft trails Apple and Google.

Microsoft will make its pitch to those developers at Build, a four-day conference on its Redmond campus, starting today. The key to Microsoft's gambit: the huge opportunity of Windows 8. Microsoft chief executive Steve Ballmer cited analyst expectations at the Windows 8 launch that400 million new PCs will be sold next year, "most of which will run Windows 8."

"The business opportunity on Windows has never been greater," said Tim O'Brien, general manager of developer and platform evangelism at Microsoft.

The Build conference comes less than a week after Microsoft brought its top brass to New York to roll out Windows 8. And yesterday, senior executives traveled to San Francisco todebut Windows Phone 8, its lagging mobile phone operating system. With the new operating systems available, Microsoft will make the case to developers at Build that now is the time for them to get on the bandwagon.

One key metric for Windows 8's success will be how much of a dent it puts into the tablet market, which Apple's iPad is dominant. Microsoft is relying on a variant of Windows 8, called Windows RT, to power many of the new tablets. Microsoft's Surface tablet, as well as offerings from partners such as Lenovo and Dell, will only thrive if developers create the wide variety of applications that consumers can already find for iPads. But developers will only create those apps if they believe the Windows 8 and Windows RT will have enough customers to merit their devoting resources to the process.

"The question is can they make a business out of it," Forrester's Rymer said. "If customers don't adopt, they're screwed."

That's why Microsoft decided to base its tablet hopes on Windows, its PC operating system, rather than Windows Phone. That's not how Apple approached the business -- it used iOS, the iPhone operating system, to run the iPad. But Microsoft is betting that the huge Windows market will lure the mobile developers that have been creating apps for Apple to its tablet operating system.

The company believes it's making progress. At Build, for example, Microsoft is attracting plenty of new developers. It said that roughly half the attendees have been to a Microsoft developer event in the last three years, compared to 65 percent at the Build conference in September 2011.

The company has also hired developer evangelists with expertise in development systems such as Node.js, used to create Web applications, in order to lure programmers who wouldn't typically consider creating apps that work with Windows.

"I want them to feel that Microsoft is truly looking out for the Web developer features they are interested in," said Rey Bango, who joined Microsoft from Mozilla in 2010, and works to promote development of Windows Store apps using JavaScript.

Some of the developers who have worked with Microsoft for years are happy to see the company reaching out to the new breed of application creators. Microsoft still has plenty of work ahead of itself to build up the Windows Store with a bevy of applications.

"They've been a punching bag for a number of years," said Chris Woodruff, who develops apps for corporate clients using Windows and co-hosts a tech podcast called Deep Fried Bytes.

At Build, Microsoft will try to punch back.