"After just four weeks on the market, it's still early to place blame on Windows 8 for the ongoing weakness in the PC market," said Stephen Baker, vice president of industry analysis at NPD. "We still have the whole holiday selling season ahead of us, but clearly Windows 8 did not prove to be the impetus for a sales turnaround some had hoped for."
NPD's data does not include Microsoft's new Surface tablet, which is only available in its own stores, nor does it take account of sales of PCs to businesses, which has recently been a much stronger market.
But if the trend is borne out over the rest of the holiday shopping season, it would be a huge disappointment for Microsoft, as well as PC makers such as Dell Inc, HP and Lenovo.
Since launch, Windows 8 accounted for only 58 percent of Windows computing device unit sales, compared to the 83 percent Windows 7 accounted for at the same point after its launch in 2009, NPD said, partly caused by poor back-to-school sales that left many Windows 7 PCs on retailers' shelves.
One patch of light for Microsoft is strong sales of touchscreen Windows 8 laptops, which accounted for 6 percent of Windows laptop sales, according to NPD.
It is still unclear how successful Microsoft's Windows 8 will be in the long term. The touch-optimized, tablet-friendly system was designed to appeal to younger users with a colorful, app-based interface, but has confused some traditional Windows customers more used to keyboard and mouse commands. Beneath the new interface design, it does not offer any radical new computing power.
On Monday, a top Windows executive said Microsoft had sold 40 million Windows 8 licenses in the month since the launch. That is ahead of Windows 7 at the same stage, but it was not clear how many of those were pre-orders, discounted upgrades, or bulk sales to PC makers.
According to tech research firm StatCounter, about 1 percent of the world's 1.5 billion or so personal computers - making a total of around 15 million - are actually running Windows 8.