For now, though, most personal computing is still performed through Windows. StatCounter, another market share tracker similar to Net Applications, reports desktop and mobile operating system share separately, instead of combining them into one statistic. The combined desktop share of Windows XP, Vista and 7 is still at 91.94%, compared to 6.25% for Mac OS X and 0.75% for Linux, .

But Windows Phone 7 doesn't even show up on StatCounter's mobile OS share ranking, which is dominated by Symbian, 's iOS, BlackBerry and .

Microsoft can expect increasing revenue for Windows, simply because PC shipments are in both 2011 and 2012, according to IDC. So it may not be that Windows is getting smaller -- it's more that the rest of the market, driven by mobile devices, is getting bigger and Microsoft isn't capitalizing on the growth. More and more users are accessing the Internet on devices that don't run Windows.

"I have no doubt that the number of Windows users has grown significantly over 2010, and am equally confident in growth in the number of users of Mac OS in 2010," Vizzaccaro says. "What we're seeing is faster growth in mobile and tablet usage. I don't think Windows Phone 7 has been on the market long enough yet to help Microsoft on the mobile front, but I do believe Microsoft recognizes how important the mobile arena is, and is focused on being successful there."

While Net Applications' data shows Windows' market share dropping from 93.74% in February 2009 to 90.29% in December 2010, Mac moved from 4.55% to 5.02% in the same time frame. Net Applications collects its data from about 40,000 websites, counting unique visitors, specifically "one unique visit to each network site per day."