Financial measures will also be closely watched. Microsoft reported an operating profit of more than $10.8 billion from Windows for PCs in its fiscal year ended June 30. That was more than half the company’s overall operating profits for the year—but down from more than $13 billion the year before.

Over the years, the Windows business has given Microsoft the financial resources to make long-term bets in areas such as video games and online services, where sustainable profits have proven much harder to attain.

Microsoft also wants to keep Windows PCs at the center of the evolving digital world. To do that, the operating system needs to work well as a hub for mobile devices and as an access point for internet services, in addition to running traditional PC software.

Windows also needs to be compelling enough to make people stick with it as web-based applications make the underlying operating system less significant in the overall computing experience.

Direct competition is also getting tougher. Macs have gained momentum in recent years—accounting for about 9.4 percent of U.S. computer shipments in the third quarter, according to research firm IDC — as Apple has introduced more people to its brand through the iPod and iPhone. Google has announced plans to introduce its own PC operating system, Chrome, next year.

“Both in terms of Windows’ strategic role going forward and its historical role in Microsoft’s revenues, Microsoft can’t afford a Windows launch that’s anything less than totally successful,” said Michael Gartenberg, vice president of strategy and analysis at Interpret LLC research firm in New York.