Anyone keeping an eye on Microsoft’s once powerful market dominance will be interested to know Windows has suffered its biggest market share dive in the last two years.

An Internet measurement company reports that the operating system’s share of the market has dipped under 90% for the first time.

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

And the winner is: Apple’s Mac OS X, which posted its biggest gain in the same two-year period, growing by 0.66% to end the month at 8.9 percent. November was the third month running that Apple’s operating system remained above 8%.

Because all new Apple computers now sport those nifty dual core processors, it’s attracting more and more adopters who need the flexibility of having Windows when they need it, but being able to use the more user-friendly Mac OS for anything else.

One bit of evidence that’s even more interesting: Net Applications finds that 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.

Tags: Apple, Mac, market share, OS, Windows