Windows 7 finished January with a 7.5 percent usage share, a mark the little-loved Windows Vista didn't attain until 11 months after its January 2007 debut.
"There's no slowing of the Windows 7 growth curve," said Vince Vizzaccaro, executive vice president with Net Applications, who also said that Microsoft 's newest OS accounted for more than 10 percent of all machines that are online.
Windows 7's share typically climbs on weekends and holidays, as more of the people online are running home computers, which are more likely to run the new operating system.
During January, Windows 7's weekly averages increased steadily from 7.2 percent to 7.4 percent, then to 7.6 percent and finally to 8 percent in the final week. For the month, Windows 7 was up 1.8 percentage points, its second-biggest one-month increase.
As in recent months, both Windows XP and Windows Vista lost share in January as Windows 7 gained ground. Windows XP slid to 66.3 percent, down 1.46 percentage points, a new single-month record that erased the 1.43-point decline of November 2009.
Vista, meanwhile, lost 0.5 of a percentage point to end at 17.4 percent. Vista has now lost usage share three months in a row, and in four of the last five months, solidifying the trend that points to a permanent decline as users abandon it for Windows 7.
If the trends of the last three months persist, Windows 7 will overtake Vista as the second-most popular operating system - and grab the No. 2 spot behind XP - in June.
This isn't the first time that NetApplications has compared the uptake of Windows 7 with its predecessor. Last November, Vizzaccaro said that Windows 7 had reached a 4 percent share in just three weeks, a mark Vista needed five months to hit.
The bulk of Windows 7's gains have come at XP's expense, not surprising since the eight-year-old-and-counting OS is the most popular on the planet by a huge margin.
Since Windows 7's late October 2009 debut, Windows XP's share has dropped nearly three times more than Vista's.
Apple's Mac OS X climbed slightly last month, finishing December with a 5.13 percent share after an increase of just 0.02 of a percentage point. It was the first gain posted by Mac OS X in NetApplications' data since October.
The small rise is no indicator of Mac sales, of course, which Apple announced last week had set a new single-quarter record of nearly 3.4 million units worldwide.
Vizzaccaro again pointed out the mobile category as the one to watch. "This is the beginning of a shift to a lot more mobile operating systems on the web," he said.
"The trend from desktop to mobile, month to month, is a curve that looks like it's going to continue to go up."
In January, mobile operating systems powered 1.5 percent of all the hardware that browsed the web during the month, Vizzaccaro said, an increase of 0.13 of a percentage point over December.
He was at a loss to explain the flattening of the growth of Google's Android operating system, although he mentioned the introduction of the Nexus One and the resulting complaints when customers weren't able to reach support.
"A lack of customer support may work okay with beta software, but not with a mobile phone," Vizzaccaro said. "No-one wants to wait two weeks for an email reply to a question about their phone."
NetApplications will start tracking Apple's iPad web usage when it starts shipping in late March, said Vizzaccaro. He was pessimistic that the tablet would show a measurable share for some time, however.
NetApplications measures operating system usage by tracking the machines that browse 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.