Apple's operating system market share slipped last month as Microsoft's Windows halted its slide for only the fourth time in the last 12 months, a research company reported.

In July, Apple's Mac OS X powered 7.76 per cent of the computers that accessed the 40,000 sites Net Applications monitors for its clients, the company said Friday. The Mac's share of the OS market was down 0.18 percentage points from June's 7.94 per cent, and was lower than May's 7.83 per cent.

Windows, meanwhile, posted a small gain in overall share, climbing 0.13 percentage points from June's 90.89 per cent to end July with 91.02 per cent.

In the last year, Windows' market share has dropped from 93.06 per cent, a decrease of nearly 2 percentage points. During the same period, Apple has boosted its operating system by 25.6 per cent.

According to Net Applications, Windows' usual slow decline stopped last month because Vista's gains more than offset the losses by Windows XP and Windows 2000. Windows XP, which was retired by Microsoft from retail and large OEM use at the end of June, dropped 0.56 per cent in July, significantly less than the 0.92 per cent and 0.95 per cent decreases for June and May, respectively.

Windows XP's July downturn was below the operating system's 12-month average of 0.93 per cent, and the second-smallest loss during the past year.

Although Apple's share has increased over time, there have been months when Mac OS X's climb has been temporarily stymied. In July 2007, for instance, the Mac's operating system fell 0.04 percentage points from the previous month. However, the June-July span last year was the last time that Apple posted share decreases for two consecutive months.

Other operating systems that Net Applications tracks were also up for the month, including Linux (up 0.02 percentage point) and the iPhone (up 0.03 points), which the research company monitors as separate from Mac OS X.

Net Applications' operating system share and trend data is available online.