On the heels of Microsoft execs crowing about selling 40 million Windows 8 licenses in its first month of availability , analysts at NPD are putting that figure more in context.

Since the Windows 8 launch on October 26, the number of Windows devices sold at retail have fallen 21 percent vs. the same period last year, according to a November 29 NPD press release. Desktop sales are down nine percent compared to a year ago; notebook sales are down 24 percent compared to a year ago.

NPD measured retail-sales data from October 21 to November 17, 2012 from a subset of a panel of retailers it surveys regularly. (NPD's numbers do not include sales of the Surface RT PC/tablet devices that Microsoft also launched on October 26.) So this is all about the consumer market -- which is Windows 8's sweet spot.

"Since its launch, Windows 8 has captured just over half (58 percent) of Windows computing device unit sales, compared to the 83 percent Windows 7 accounted for four weeks after that launch. 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," according to NPD's press release.

NPD noted that there was a lof of Windows PC inventory left in the channel after the back-to-school season (which Microsoft missed with Windows 8 ), and that has had an impact on the initial sell-through rates of Windows 8.

NPD didn't mention the fact that there aren't a whole lot of Windows 8 showcase devices available, even though it's been a month since launch. There are next-to-no Intel "Clover Trail" devices in the market yet. Windows Chief Financial/Chief Marketing Officer Tami Reller acknowledged during an appearance at a tech conference earlier this week that more than a few of the expected big sellers -- Windows 8 machines from Dell, Lenovo and HP -- are just coming to market now or in a matter of weeks.

Microsoft officials won't say what they are counting with that 40 million figure. They won't say if this number includes both sales to the channel and sales to consumers; how many of those copies are upgrades vs. preloads on new machines; and how many of these copies are in use/activated. But it's likely a good percentage of these 40 million are sales to OEMs, given NPD's numbers, I'd say.

Update: Reader @steveymacjr went back into the archives and found NPD's analysis from the first month of Windows 7 sales. Microsoft sold more "standalone Windows 7 software units" in its first week of sales than it did with Vista, but Windows 7-based PC sales were lower than they were with Vista, NPD found. So maybe this is just history repeating itself (?).

I do remember I wasn't too happy with the selection of Windows 7 PCs in retail stores at launch. I thought this time might be different. This time around, however, Clover Trail's late arrival seems to have contributed to the lack of compelling new Windows 8 PCs and tablets available right out of the gate.