Microsoft on Monday announced that it sold more than 4 million copies of the Windows 8 upgrade in the system’s first three days of availability. This is the first step toward the firm’s stated goal of reaching what it calls "the next billion users."

The announcement came during CEO Steve Ballmer’s appearance at the Microsoft BUILD keynote on Monday. The event is being held at the Microsoft campus in Redmond, Washington.

Microsoft used the Day 1 keynote to communicate the Windows 8 developer opportunity, which spans 300 to 400 million new users in the coming 12 months, plus an upgrade base of more than 670 million existing Windows 7 users. Combined, this is the largest audience for any platform, including newcomers like Google Android and Apple iOS.

But Windows 8 comes with some challenges, for both users and developers. It features both the classic desktop interface and a new mobile interface formerly called Metro, which can be confusing to customers. And developers who have followed Microsoft down various technological dead ends like Silverlight might be leery of yet another new developer platform.

Worst of all, Microsoft’s Windows Phone platform has foundered in the market. The hope is that the new version, based on Windows 8, will reverse this trend.

I’ll be closely tracking Windows 8 and new Windows device sales in the weeks and months ahead. I’m very curious to see how this system performs, compared with both its Windows 7 predecessor and the new classes of mobile devices with which it now competes.

But 4 million is a good start.