Microsoft Windows continued its long term gradual slide as the operating system's usage share dropped by about a third of a point. That was despite Windows 7 posting a second straight month of impressive gains, according to web metrics firm Net Applications.

Although rival desktop operating systems - Mac and Linux - essentially remained flat, mobile operating systems, including Google Android and Apple iPhone OS, took up the slack created by the Windows dip. Mobile operating systems, said Net Applications, now power 1.3 per cent of all the hardware that surfs the internet.

Windows finished the year with a 92.2 per cent 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, the 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.

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

Windows XP slid 1.3 percentage points in December, its second-largest one-month decline ever. (The record remains November, when XP lost 1.4 points.) Vista, meanwhile, lost 0.7 of a percentage point, a single-month record, to end at 17.9 per cent. December was the second month in a row that Vista lost share, and the third in the last four months, a trend that points to a permanent decline as users abandon it for Windows 7.

Still, the bulk of Microsoft's losses since the Windows 7 launch have been from Windows XP; the eight-year-old OS has lost 2.7 points in the last two months, while Vista has lost only one point.

Microsoft's newest OS, on the other hand, boosted its share by 1.7 percentage points to end December with 5.7 per cent, meaning that approximately one out of every 18 machines on the Web ran Windows 7 last month. If it can keep up the pace of the last 60 days, Windows 7 will crack 7 percent this month, beating Vista to that number by six months.

Windows 7 also reached a milestone on January 1, 2010, when it posted an eight per cent share for the day. The previous one-day record of 7.6 per cent had been set on December 27, 2009.

Apple Mac OS X dipped for the second month in a row, finishing December with 5.1 per cent after a decline of a statistically insignificant 0.01 of a percentage point. Most months, however, Mac OS X posts gains, not losses: December was only the fifth month of 2009 in which Apple's operating system lost share.

The winner, according to Net Applications: mobile operating systems, which accounted for 1.3 per cent of all operating systems powering devices that browsed the internet in December. Although their shares remained small - the largest was Java Platform, Micro Edition, with just 0.53 per cent, followed by the iPhone OS with 0.44 per cent - month-over-month increases were dramatic in some cases. Google Android operating system, for example, increased its share by nearly 56 per cent between November and December, while RIM and the iPhone boosted their shares by 22 per cent and 20 per cent, respectively.

Net Applications measures operating system usage by tracking the machines that surf to the 40,000 sites it monitors for clients, which results in a pool of about 160 million unique visitors each month. It then weights share by the estimated size of each country's internet population.

December's operating system data can be found on Net Applications' site.