After a February market share gain, March saw Windows market share drop again, when more users jumped ship from Windows XP than picked up Vista, according to Web metrics firm Net Applications.

Apple Mac OS X, on the other hand, gained market share for the first time since January, the firm reported. In March, Net Applications reported that slightly more than 88 percent of users that connected to Web sites monitored by the firm used Windows-powered computers to do so. Though the percentage represents a drop of 0.28 of a percentage point from February, it is the smallest drop for Windows since June 2008.

In February, Windows gained more market share than it had in more than 18 months, according to Net Applications.

Windows XP, which lost market share for the 21st consecutive month, contributed to the March downturn, sliding 0.82 of a percentage point and accounted for 62.9 percent of the machines that surfed to Net Applications' sites in March.

While Windows Vista boosted its share, running 23.4 percent of the machines connected to the Web in March, the increase of 0.63 of a percentage point wasn't enough to make up for the loss of Windows XP users. If the operating system continues to gain ground at its 12-month average pace, it will break the 25 percent mark in May, Net Applications reported.

The beta version of Windows 7, however, gained only 0.03 of a percentage point in March, less than half the increase it posted the month before, to end with a 0.21 percent share. With the public beta no longer available from Microsoft, it's likely that the in-development operating system stalled because it can be found only on file-sharing sites that host pirated copies of post-beta builds.

For Mac OS X, March saw the operating system regain half of what it had lost the month before, increasing 0.16 of a percentage point to end with a 9.8 percent market share. If Apple’s OS continues on its current 12-month average pace of growth, it should come close to cracking the 10 percent mark by the end of April, Net Applications reported.