Although rival desktop operating systems -- Mac and -- essentially remained flat, mobile OSes, including 's Android and 's iPhone OS, took up the slack created by Windows' dip. Mobile operating systems, said Net Applications, now power 1.3% of all the hardware that surfs the Internet.

Windows finished the year with a 92.2% share, down 0.3 of a percentage point. It was the eighth month in 2009 during which Windows lost share.

As it did in 2008, Windows' decline again accelerated in the second half of the year, when it lost 1.2 points of share. That compared to a drop of just 0.5 of a percentage point in the first six months of 2009. In 2008, Windows also lost more than twice as much share between July and December as it did in the preceding six months.

But the slip doesn't mean Windows is in any danger of losing its grip on the operating system market anytime soon: At the pace of the last 24 months, Windows would retain a majority share for another 27 years.

, both Windows XP and Windows Vista lost share in December, while gained ground. Unlike in November, however, Windows 7 was unable to make up for the decline in 's older operating systems.