Research by the web metrics firm showed that for each copy of Vista replaced by Windows 7 during November, more than six copies of XP were swapped for the new OS.
Meanwhile, Apple's Mac OS X lost share during November.
But it was Microsoft's ability to retain its share in the face of record slumps in its older editions that was the biggest revealtion from Net Applications.
Even though Windows XP lost 1.45 percentage points to end November with a 69 percent share, and Vista fell 0.2 percentage points to 18.6 percent, Windows kept its total operating system share at 92.5 percent, the same as in October.
The declines in XP and Vista were both records in Net Applications' tallies, which because of a change in methodology instituted last July go back only two years.
For Vista, November marked the second time in three months that the often-maligned operating system lost share. That trend, if accurate, means that the 2007 operating system has peaked, and will now, like XP before it, begin a slow, inexorable decline as it is replaced by Windows 7.
Give Windows 7 all the credit for holding Microsoft's line. In the first full month after its October 22 public launch, Microsoft's newest operating system increased its share by 1.8 percentage points, ending November with 4 percent, more than enough to make up for the losses by XP and Vista.
Windows 7 has been on a share roll since it debuted, according to Net Applications. Less than three weeks after its release, Windows 7 had acquired a slice of the OS pie that it took Vista five months to reach.
Neither XP nor Vista will vanish overnight if Net Applications' data is any indicator. Currently, about three of every four Windows PCs runs XP, while one-in-five runs Vista. Only about one in every 23 Windows systems is powered by Windows 7.
Almost as unusual as Windows remaining in place was the Mac OS X's dip. By Net Applications' estimate, Apple's operating system finished November with 5.1 percent, a decline of 0.16 percentage points, the largest since February 2009 and only the third negative number this year.
Most months, Mac OS X gains ground on Microsoft, albeit by small margins: Over the last 12 months, Apple's OS has increased its share by an average of less than 0.1 percentage points.
Linux, on the other hand, came up a winner last month, returning to the 1 percent share mark for the first time since July. Linux's all-time high in Net Applications' rankings was May 2009, when it nearly reached 1.2 percent.
Net Applications measures operating system usage by tracking the machines that surf to the 40,000 sites it monitors for clients, which results in a pool of about 160 million unique visitors each month. It then weights share by the estimated size of each country's internet population.
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