Consumers are flocking to Windows 8. Microsoft has sold more than 4 million Windows 8 upgrades in its first three days, chief executive Steve Ballmer announced this morning.

Ballmer revealed the figure during his opening keynote at the Build developer conference — which we’re calling Microsoft’s most important event ever — in Redmond this morning. He also noted that Microsoft has sold “tens of millions” of Windows 8 upgrade units to corporate customers, who can upgrade when they see fit. Windows 8 officially launched last Friday, October 26.

Speaking at Nokia’s Lumia 920 launch in September, Ballmer boldly predicted that we’ll see 400 million Windows 8 devices next year.

Last week, Microsoft’s Windows head Steven Sinofsky announced that the company has sold more than 670 million Windows 7 units since its release in 2009. So far, Windows 8 is off to a strong start — though I have a feeling many consumers will choose to stick with Windows 7 instead of upgrading. From my experience, Windows 8 works best on new hardware with touchscreens or modern touchpads — on a desktop or older laptop, Windows 8 doesn’t really have much to offer beyond Windows 7.

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