A week after Microsoft took the wraps off Windows Longhorn, software developers are cogitating on one key question: What's the killer app?

At its Professional Developers Conference in Los Angeles last week, Microsoft released to 7,000 developers a technical preview version of Longhorn, the code name for a new version of the Windows desktop operating system that's due in three years. It's a long lead time for testing a product, but then, there's a lot of new technology in Longhorn.

"We're trying to acclimate developers to our concepts, so by the time we're ready to ship beta-quality bits, they're in the swing of things," says Microsoft chief technical officer Craig Mundie. A beta is due next year. Assuming it takes a couple of years to develop apps on Longhorn's programming model, releasing an early version "increases the probability that lots of interesting apps will be ready to ship with Longhorn," he says.

Microsoft released a preview of its Longhorn operating system to 7,000 developers last week; Gates says technology hasn't reached its limits.

Longhorn is a "platform shift" on the order of the move from early versions of Windows to Windows 95, or the advent of the Web browser, Mundie says. Microsoft is introducing the first new Windows application programming interface in a decade, called WinFX, which will be used to program everything in the Longhorn operating system, including three technologies called Avalon, Indigo, and WinFS. "We only get a new platform in this industry every 15 to 20 years," Mundie says. "We're not just showing the next instance of our operating system but signaling a move in programming. New platforms are driven from the grassroots level by a couple of killer apps."

What those will be is a big, open question. Microsoft needs Longhorn to succeed to improve PC sales, which are forecast to grow just 8% this year, according to market researcher Gartner. That's hurting growth at Microsoft, which gets about two-thirds of its revenue from desktop versions of Windows and sales of its Office suite. Windows revenue didn't budge from a year ago in Microsoft's first quarter ended Sept. 30, and Office sales inched up just 1%. With the delivery of Longhorn's three subsystems--Avalon, a slick new presentation graphics layer; Indigo, built-in Windows technology for writing secure, reliable Web services; and WinFS, a relational database file system that's supposed to make searching for and grouping documents more natural--Microsoft is trying to persuade its vast developer market to come up with fast, interactive client apps that interact more seamlessly with data from the Net.

"Most people are seeing servers doing all the processing and the browser being a dumb terminal," says Kevin Lynch, chief technology officer at Macromedia Inc. The Web is "great for viewing documents, but it doesn't take advantage of your personal computer as well as it could." By year's end, Macromedia plans to release Central, an application development environment for building smart-client PC apps that consume online data. Longhorn could make writing those apps easier, Lynch says.

Using WinFX, developers could use just a few commands to call search features in WinFS, vector graphics capabilities in Avalon, or Web services in Indigo. Amazon.com Inc. last week demonstrated a prototype application built using the new APIs that downloads catalog data about digital cameras to a user's hard drive for graphical searching and grouping of products, then sends orders back to Amazon's site. Longhorn could provide new capabilities for developers whose sites connect with Amazon's store, says Amazon chief technology officer Allan Vermeulen. Amazon collects a finder's fee on sales through those sites and may charge developers to license its software in the future.