Microsoft CEO Steve Ballmer has announced that Windows 7 has sold more than 400 million licenses so far, meaning it is still the fastest-selling operating system. Ballmer revealed the number at the Microsoft's annual Worldwide Partner Conference (WPC), a four-day event that celebrates the accomplishments of the company's 640,000 global partners. Windows 7 has been available to the masses for 21 months.

Windows 7 actually grabbed 20 percent of the market at the end of 2010. Between January and December 2010, Windows XP fell almost 10 percentage points to 56.72 percent market share, Windows Vista lost over five percentage points to 12.11 percent, and Windows 7 gained over 13 percentage points, pushing it to 20.87 percent.

In June 2011, Windows 7 had 27.13 percent of the market. By the end of the year, I would expect 1 in 4 users to be using Microsoft's latest and greatest, possibly even reaching the 40 percent market share mark. Meanwhile, Windows Vista is already below the 10 percent mark and Windows XP should soon drop below 50 percent.

Three months after release, Microsoft said more than 60 million Windows 7 licenses were sold. At the 15 month mark, the company had sold 300 million licenses. After 18 months of availability, the software giant had sold 350 million licenses.

The timing for the 350 million and 400 million milestones show that sales have slowed down a tad (previously they were holding steady at some 20 million licenses sold per month). Still, back to school season is coming up, followed by the holiday shopping frenzy, so the company may be able to get that number back up.

If you're already on Windows 7, you should check out our guides for Microsoft's latest operating system. If you haven't upgraded to it yet, tell us why in the comments below.