Microsoft's Windows 7 cracked the 20 percent share mark last month, a milestone the problem-plagued Vista never reached.
Statistics provided by Net Applications put Windows 7's online usage share at 20.9 percent in December, up 1.2 percentage points from the month before.
Windows Vista, meanwhile, fell by half a point to 12.1 percent, its lowest share since July 2008. Vista peaked at 18.8 percent in October 2009, the same month that Microsoft launched Windows 7.
Overall, Windows' usage share slipped by half a point to 90.3 percent, down nearly two points during 2010.
Most of Windows' losses last month translated into gains for mobile operating systems, as Apple's iOS boosted its share by three-tenths of a percentage point in December and Google's Android increased by one-tenth of a point.
Mac OS X stayed flat at 5 percent last month.
Among the editions Microsoft still supports, Windows XP fell the farthest during December, losing 1.2 points to end the year with a 56.7 percent share.
Windows XP's losses continued to accelerate as its share plunged by 3.3 points in the fourth quarter. By comparison, the nine-year-old operating system lost 2.4 points in the third quarter of 2010 and just two points in the second quarter.
Microsoft has been urging XP users to dump the aged OS for Windows 7, and analysts have said that the message has hit home: According to an October survey by Dimensional Research, more than a third of enterprises have already implemented a partial migration to Windows 7, while about one-in-17 firms have moved all their machines to the new operating system.
Even so, it's likely that XP will remain on PCs for years to come. Net Applications' newest data indicated that if Windows XP continues to lose share at the average pace of the last three months, it won't dip under 50 percent until the third quarter of this year, and will still account for 12.6 percent of all OSes in the second quarter of 2014, when it's slated for retirement from support.
Assuming Microsoft launches the next version of Windows, for now dubbed Windows 8, three years after Windows 7's debut, the latter should peak at about 44.7 percent in October 2012.
The three-year development cycle for Windows - a plan Microsoft executives have promised numerous times since the introduction of the delayed Vista - means that it's very unlikely any single edition will ever match the monopoly Windows XP has enjoyed. According to Net Applications, XP had an 83.6 percent share in November 2007, the first month it kept tabs on the operating system after rejigging its numbers to weight share by each country's online population.
Net Applications calculates OS usage share from data acquired from the 160 million unique visitors who browse the 40,000 websites it monitors for clients. The company's December operating system statistics are available on its site.