Windows 8 is selling at about the same rate Windows 7 did when that operating system, the best-selling in Microsoft's history, rolled out in late 2010, a Microsoft official said Tuesday.

Tami Reller, CFO of Microsoft's Windows division, said the company has sold 40 million Windows 8 licenses since it launched the OS on Oct. 26. "The 40 million is roughly in line with Windows 7," said Reller, speaking at the Credit Suisse Technology Conference, in Scottsdale, Ariz. Microsoft has previously said that it sold 60 million Windows 7 licenses during that operating system's first three months on the market.

Reller also said Windows 8 upgrades are outpacing Windows 7 upgrades that occurred during the latter's first 30 days of availability. "Windows 8 upgrade momentum is outpacing that of Windows 7," she said.

Reller's comments were surprising in that numerous market watchers have indicated that Windows 8 is off to a sluggish debut. Last week, Topeka Capital Markets analyst Brian White said that his checks of the Asian supply chain revealed that Windows 8 is off to a slow start. "Much lower than ...PC makers originally expected a few months ago," said White, in a report.

Also last week, Deutsche Bank cut its estimate for PC sales in the current quarter, due to "lackluster initial uptake of Windows 8," according to analyst Chris Whitmore.

[ Buy yourself a little holiday gift. See 8 Cool Windows 8 Apps: $8 Or Less. ]

Microsoft didn't specify what percentage of the 40 million licenses sold were from upgrades, direct sales to end users or sales to PC makers who preinstall the OS on hardware, some of which may still be sitting on store shelves or in warehouses.

Reller seemed to suggest that Microsoft is prepared for, or is at least willing to tolerate, a slow start for Windows 8. "Windows 8 represents really a generational shift of hardware, a generational shift of the operating system and apps, all together, all at once," said Reller. "What I think is important to mention is it was built for the future, not just any one single selling season."

Reller is one of two Microsoft execs overseeing the company's Windows group in the wake of the sudden departure earlier this month of unit president Steven Sinofsky. The other is Julie Larson-Green, head of Windows software and hardware engineering. Reller said Sinofsky's exit would not impact Windows 8's future success.

"We have the benefit of such a strong leadership team and oftentimes in Windows you really only get to see a few visible senior leaders, but the reality is, not only are those senior leaders very strong, but [so is] the bench of senior leaders that represent the product holistically," Reller said.

Microsoft shares were off 0.5%, to $26.94, in morning trading Wednesday.

Upgrading isn't the easy decision that Win 7 was. We take a close look at Server 2012, changes to mobility and security, and more in the new Here Comes Windows 8 issue of InformationWeek. Also in this issue: Why you should have the difficult conversations about the value of OS and PC upgrades before discussing Windows 8. (Free registration required.)