Windows 10's cut of Web traffic grew last month but not as much as in August.

Windows 10 continued to grab a larger audience in September, though it seems to have lost some steam.

The latest version of Windows snagged a 6.6 percent cut of all Web traffic generated by desktop operating systems last month, according to NetMarketShare, less than 1.5 points higher than the 5.21 percent seen in August. Windows 10 had been available as a technical preview since October 2014 and as such barely registered in NetMarketShare's data until it rolled out as a free upgrade starting July 29.

Windows 10 success is critical to Microsoft. The company dropped the ball three years ago with Windows 8, which failed to win the hearts and minds of many desktop users. To bring back interest, Microsoft revamped its new OS by reviving the Start menu, making the interface more keyboard and mouse friendly and adding its Cortana voice assistant. But the major lure has been to make Windows 10 a free upgrade for the first year to anyone running Windows 7 or 8.1. After that, Windows 10 jumps to its full price of $119 for the Home edition and $199 for the Pro version. So Microsoft needs to see strong growth in adoption during these first 12 months.

The data from NetMarketShare for August appeared to show that people were taking advantage of the free upgrade offer in droves. But the number for September showed growth in demand rising only slightly. It's hard to say if the small gain is the start of a trend following a quick start out of the gate or simply a blip for the month of September.

Indeed, other versions of Windows showed little movement last month.

In first place, Windows 7's share actually fell to 56.5 percent from 57.6 percent in August. In second place, Windows XP was relatively flat, with its cut nudging up to 12.2 percent from 12.1 percent. In third place, Windows 8.1 declined to 10.7 percent from 11.3 percent. And Windows 8 was also flat with a share of 2.6 percent.

Based on NetMarketShare's data, September seemed like a month in which a lot of people simply stayed put with their current version of Windows. That could be due to the season, as September is the month in which kids go back to school and people return to work from vacations, so they may have more on their plates than jumping to a new version of Windows. Numbers for September 2014 also showed just minor changes in Web traffic from the prior month for the different flavors of Windows.

Overall, Windows 10 is off to a good start compared with its predecessor. Adoption of the new OS is already far outshining that of Windows 8 over a similar time frame. After Windows 8 officially launched on October 27, 2012, it took home just a 1 percent share of Web traffic in November and just 1.7 percent in December, according to NetMarketShare. It didn't surpass 6 percent until the following August when it scored a 7.4 percent share.