Windows has suffered its biggest US market share dive in the last two years, an Internet measurement company has reported, sending the operating system's share under 90 percent for the first time.

In November, 89.6 percent of users who connected to the websites that Net Applications monitors did so from systems powered by Windows, a drop of 0.84 of a percentage point from October. The decrease was the largest slip by Windows in the last two years, and easily bested other recent down months, including May 2008 and December 2007, when Windows lost 0.51 and 0.63 percentage points, respectively.

Apple's Mac OS X, meanwhile, posted its biggest gain in the same two-year period, growing by 0.66 percentage point to end the month at 8.9 percent. November was the third month running that Apple's operating system remained above 8 percent.

Vince Vizzaccarro, Net Applications' executive vice president of marketing, attributed Windows' slip to some of the same factors he credited with pushing down the market share of Microsoft's Internet Explorer (IE) browser.

"The more home users who are online, using Macs and Firefox and Safari, the more those shares go up," he said. November was notable for a higher-than-average number of weekend days, as well as the Thanksgiving holiday in the US, he noted.

Windows' share typically falls on weekends and after work hours, as users surf from home computers, a larger percentage of which run Mac OS X than do work machines.

Notable in Windows' downturn was a dramatic drop in share of the aged Windows XP - the largest decrease since January 2008 - and a major uptick in Windows Vista's share. While XP lost 1.81 percentage points, Vista gained back 1.16 points of that, its largest move since last January.

Windows 2000, the only other edition that Net Applications tracks, continued its slide toward 1 percent, falling to 1.56 percent during November.

As expected, Vista cracked the 20 percent mark for the first time last month, ending November with a 20.45 percent share.

Windows' share shows no sign of stopping its slow slide; in the past 12 months, Microsoft's market share has fallen from 91.79 percent, a decrease of more than 2 percentage points. During the same period, Apple has increased its operating system market share by 1.56 points, or a gain of 21.3 percent.

Net Applications also noted a small boost in market share for the open-source Linux operating system, which grew from 0.71 percent in October to 0.83 percent last month. In August and September, however, Linux had a share above the 0.90 percent mark.

Operating system market share data is available online at Net Applications' site.