By Shira Ovide 

Microsoft CEO Satya Nadella spent his first year on the job making the company less dependent on Windows. This week, when Mr. Nadella is scheduled to show off the next version of Microsoft's most famous product, will reaffirm the operating system's key role in his company's fate.

At a day-long press session Wednesday, Mr. Nadella and other Microsoft officials are expected to give the most detailed glimpse so far of Windows 10, slated to launch later this year.

Microsoft also will explain how Windows will work more seamlessly with upgrades to the popular Office productivity software bundle and the Xbox videogame system, and it may offer views of a voice-activated digital assistant and a new mobile Web browser.

Windows is no longer the beating heart of Microsoft, and Mr. Nadella has won plaudits for stressing other products such as Office and Azure, a service for corporate cloud computing. But the operating system remains a linchpin of Microsoft's strategy, and the direct or indirect source of an estimated 80% of the company's profit. That makes the first Windows launch on Mr. Nadella's watch a fraught assignment.

The stakes are particularly high for Windows 10, which must solve several problems for Mr. Nadella. Microsoft must recover from missteps from the previous version, establish a gateway for new add-on services, and help salvage its struggling smartphone business.

In prior previews of Windows 10, the company acknowledged it went off the rails with some elements of the earlier version, called Windows 8. (Microsoft unaccountably skipped Windows 9.) Microsoft is de-emphasizing a screen offering smartphone-like apps, introduced in Windows 8, that confused some users with its fusion of mobile and PC conventions. Microsoft officials will continue to stress that Windows 10 is more like the Windows that people have come to expect.

"What Microsoft has got to do is continue to provide a stable, familiar-to-use version of Windows," said David MacDonald, chief executive of Softchoice Corp., which sells technology and services to companies. Mr. MacDonald said few of his corporate customers installed Windows 8 on workplace PCs, preferring to stick with prior versions.

Microsoft also hopes Windows 10 will fulfill its long-standing promise to establish a common software foundation for PCs, tablets, smartphones and Xbox. Those products now require four fairly distinct operating systems. Unifying them could allow for seamless activities across devices, such as letting Xbox users shift in midgame from the console to a Windows PC.

The larger benefit of a cross-device operating system, however, is creating a bigger base of users to appeal to software developers--a crucial constituency for Microsoft's flailing smartphone business. Microsoft's Windows Phone software is used on just three out of 100 new smartphones sold world-wide, and one reason is a lack of popular or buzzy mobile apps. Microsoft hopes to burnish its allure to developers with the promise that apps created for Windows phones will also run on hundreds of millions of Windows PCs.

Windows' role in generating revenue makes every new version a high-stakes venture. The PC operating system accounted for about 19% of Microsoft's revenue in the year ended June 30, and it generates roughly 30% of the company's earnings, Nomura Securities stock analyst Rick Sherlund estimates.

Those figures actually understate the operating system's financial potency, because sales of Office are closely tied to sales of Windows devices. The sum of revenue related to Windows and PC versions of Office accounts for roughly 80% of Microsoft's operating profit, according to estimates from Jefferies & Co.

Windows 10 may usher in some business-model tinkering. Microsoft executives have hinted the company will experiment with new ways to make money, perhaps by pitching people on add-on services or apps, such as Microsoft's Skype video-calling service, OneDrive file storage and digital video downloads.

Some analysts have also speculated that Microsoft could give some people the option to buy Windows 10 as an annual subscription, rather than a one-time purchase. That could shift the Windows revenue engine into overdrive.

The Week Ahead looks at coming corporate events.

Write to Shira Ovide at shira.ovide@wsj.com

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