Microsoft on Tuesday announced that it has sold 40 million Windows 8 licenses since the system's release last month.

Windows CFO Tami Reller shared the news yesterday at the Credit Suisse 2012 Annual Technology Conference. "The journey is just beginning, but I am pleased to announce today that we have sold 40 million Windows 8 licenses so far," she said.

In a followup blog post, Microsoft said "Windows 8 is outpacing Windows 7 in terms of upgrades. We built Windows 8 to work great on existing Windows 7 PCs. And we also set out to make upgrading from Windows 7 to Windows 8 super easy."

More than five years ago, when Microsoft unleashed Windows Vista, the company said it sold 20 million licenses during its first month. "While it's very early in the product lifecycle, we are setting a foundation for Windows Vista to become the fastest-adopted version of Windows ever," Bill Veghte, a former Windows exec now at HP, said at the time.

Windows XP sold just over 17 million licenses after two months, Microsoft said in 2002.

The day after Microsoft announced initial Vista sales, however, watchdog site Microsoft Watch quibbled with Redmond's numbers. The site reported that Redmond used the Vista-versus-XP comparison "to bolster its Vista gangbusters sales claim." Microsoft Watch's Joe Wilcox said that "by my accounting, Vista is actually off to a slower start than Windows XP, using real world comparisons."

Microsoft released Windows 7 in Oct. 2009, and sold 90 million licenses by March 2010. CFO Peter Klein then said at the Morgan Stanley Technology, Media & Telecom Conference that Windows 7 was "the fastest selling operating system in history."

Again, critics were skeptical. NPD reported that Microsoft moved more standalone Windows 7 software units in its first week than it did with Vista, but that Windows 7-based PC sales were not as high as they'd been with the previous OS.

For more, see PCMag's review of Microsoft Windows 8 and the slideshow above.