Microsoft’s Windows operating system was running on about 62% of all computers worldwide in December 2012 according to online advertising network Chitika. Apple iOS (iPhone and iPad) was the second most popular OS with a market share of 15.80 percent.
Apple's Mac OS X was third with 9.43 percent, followed by Android with 8.43 percent of the market share.
Note: Though I posted this in Mobile OS forum, but the Windows OS referred above is the desktop version of Windows (Windows XP, 7, 8), not the Windows Mobile OS.