Microsoft is boasting that it sold 40 million licenses for its newest operating system, Windows 8, in its first month after launch on Oct. 26.

But research firm NPD Group is unimpressed, saying on Thursday that sales of all Windows-based devices fell 21 percent last month, even with the "boost" from the brand new system. The figures are based on weekly data Relevant Products/Services supplied by retailers.

"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, in a statement. "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."

Touchscreen Notebooks Hot

NPD said Windows 8 devices comprised 58 percent of overall Windows device unit sales, compared with the 83 percent that Windows 7 accounted for four weeks after its launch. Windows notebooks with touchscreens, however, accounted for 6 percent of Windows 8 notebook sales at an average price of $867.

NPD called Windows 8 tablet sales "almost non-existent, with unit sales representing less than 1 percent of all Windows 8 device sales to date." However, in a footnote, the company said its figures did not include the heavily advertised flagship Windows tablet, Microsoft's Surface.

NPD did not respond to messages seeking an explanation of the omission in time for publication.

Baker said back-to-school sales of Windows devices "left a lot of inventory in the channel, which had a real impact on the initial sell-through rates for Windows 8." He said the average cost of a Windows computer jumped significantly this year from $433 to $477, while Windows 8 notebooks have seen a nearly $80 rise in selling prices versus the prior year.

Charles King, principal analyst at Pund-IT, told us that while NPD's analysis is bleak, it should be understood that Windows 8, with its mobile device integration and touchscreen orientation, is a radical change in the computing experience and will naturally take longer to catch on than its predecessors.

Breaking-In Period

"In essence, the company has completely reimagined the Windows experience, meaning that it'll take users longer than usual to get used to the OS -- and decide whether to opt-in with their wallets," King said. "So yes, sales are off but given the nature of the Windows 8 beast that seems hardly surprising.

King said he is not prepared to jump on the bandwagon with those who believe the PC era is over. "In fact, the design of W8 shows that the PC's largest proponent recognizes that personal computing has changed fundamentally and is simply no longer device- or location-centric," he said.