Unit sales of Windows PCs and tablets to consumers in the U.S. are down 21 percent since the Windows 8 launch a month ago, compared with the same period last year, according to new numbers released this morning by the NPD Group market research firm. NPD notes in a news release that Windows 8 tablet sales “have been almost non-existent, with unit sales representing less than 1 percent of all Windows 8 device sales to date.”

The PC market overall has been struggling this year, so the declines are a continuation of that trend and a sign that Windows 8 isn’t yet providing a lift to the U.S. consumer PC market.

One caveat: NPD’s numbers don’t include sales of Microsoft Surface. Microsoft is selling its new tablet exclusively in its own retail and online stores for now.

One bright spot, according to NPD analyst Stephen Baker: “The strong performance of Windows 8 notebooks with touchscreens, where Windows 8 truly shines, offers some reason for optimism. These products accounted for 6 percent of Windows 8 notebook sales at an average price of $867 helping to re-establish a premium segment to the Windows consumer notebook market.”

In the month since its launch, Windows 8 represented 58 percent of Windows device sales to consumers in the U.S., compared with 83 percent for Windows 7 during the comparable time period following its launch three years ago. A difficult back-to-school shopping season gets part of the blame, leaving lots of Windows 7 inventory for retailers to sell over the past month.

Earlier this week, Microsoft disclosed Windows 8 sales figures for the first month since the operating system’s debut, announcing that 40 million licenses had been sold to businesses and consumers worldwide. A report on Nov. 16 by longtime Microsoft watcher Paul Thurrott said Windows 8 sales were well below Microsoft’s internal projections, citing an anonymous source.

Here’s the full rundown from NPD.

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