If you consider NetApplications' data set, then Linux owns only about 1 percent of the desktop OS market and Windows has almost 92 percent. But if you consider all computing platforms, including mobile, than Windows has only 20 percent and Linux has 42 percent - and that would be in the form of Google's Android alone.
Goldman Sachs recently published a chart which shows the shift from Microsoft's 95 percents hare of the computing platform market in 2004, when PCs dominated the computing landscape, to just 20 percent in 2012. The forecast suggests that Microsoft will be able to grow its share back to 26 percent by 2016 and Android will shrink to 39 percent, while Apple's iOS and MacOS X will expand from 24 percent today to 29 percent in 2016.
Much of those projections are based on crystal ball evaluations and depend especially on Microsoft's ability to establish Windows Phone and Windows RT against Android an iOS.