For the second time in three months, Windows has boosted its market share, an Internet measurement company reported today.

The increase by the Microsoft Corp. operating system was the largest in over a year, said Net Applications Inc.

During October, 90.46% of users who connected to the Web sites that Net Applications monitors did so from systems powered by Windows. The share was up 0.17 percentage point from September and was the biggest gain since June 2007, when Windows climbed by 0.4 point.

Apple Inc.'s Mac OS X, meanwhile, failed to grow its share for just the fourth time this year. The decline was extremely small, just 0.02 percentage point. Apple's operating system remained above 8% for the second month running, however.

Mac OS X tends to slip slightly each October, according to Net Applications. Last year, for example, the OS slid 0.05 point that month.

Windows' overall gain appeared to come courtesy of a strong showing by Vista, which increased its market share by more than the other editions -- notably Windows XP and Windows 2000 -- lost.

According to Net Applications, Vista added 0.96 percentage point last month to end at 19.29%, while XP and 2000 lost 0.56 and 0.17, respectively. Previous increases in total Windows share, including one in August, have also been pegged to stronger-than-average gains by Vista.

Vista has yet to crack the 20% share mark, although if past performance is any guide, it will do so shortly. The operating system's share surged during November and December 2007, and in January 2008 -- likely because of new purchases of PCs by consumers during the holiday season.

Over the long term, however, Windows' share has declined. In the past 12 months, Microsoft's market share has fallen from 92.42%, a decrease of nearly 2 percentage points. During the same period, Apple has increased its operating system market share by 1.41 points, or 20.7%.

Net Applications also noted a dip in market share for the open-source Linux operating system, which dropped from 0.91% in September to 0.71% last month.

The company's operating system market share data is available online.