By the end of 2010, Windows 7 had grabbed 20 percent of the market. At the end of April 2011, Microsoft can already say that Windows 7 has passed the 25 percent mark, according to Net Applications. In other words, the latest and greatest operating system from Redmond is now installed on one in four computers.

While Windows 7 is surging in market share, Windows is slowly slipping: between March and April, it fell 0.67 percentage points (from 89.58 percent to 88.91 percent). At the same time, Mac OS gained 0.15 percentage points (from 5.25 percent to 5.40 percent) and Linux fell 0.02 percentage points (from 0.96 percent to 0.94 percent). Unsurprisingly, mobile operating systems gained share.

In the same time period, Windows 7 gained 0.94 percentage points (from 24.17 percent to 25.11 percent). Windows Vista meanwhile slipped 0.34 percentage points (from 10.56 percent to 10.22 percent) and Windows XP fell 1.21 percentage points (from 54.39 percent to 53.18 percent). It won't be long now before we see Windows Vista in single digit market share and Windows XP below the 50 percent mark.

Last month, Microsoft announced that Windows 7 had sold more than 350 million licenses in the last 18 months. This means that Microsoft is selling just fewer than 20 million licenses per month. Back to school season is coming up, followed by the holiday shopping frenzy, so that number should remain stable till 2012 at least.

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.