Microsoft's new Windows 8 has not given a boost to U.S. sales of PCs and tablet computers, according to the NPD Group.

Since the highly anticipated operating system was launched Oct. 26, Windows device sales have fallen 21 percent compared with the same period last year, the market research firm said.

Notebook sales, which have been weak throughout most of 2012, were down 24 percent, and desktop sales dropped 9 percent.

Through Nov. 17, Windows 8 had captured just 58 percent of Windows computing-device unit sales. That compared with 83 percent for Windows 7 in the four weeks after that operating system's debut, NPD said.

Windows 8 tablet sales have "been almost nonexistent," with unit sales representing less than 1 percent of all Windows 8 device sales to date, NPD said Thursday.

"After just four weeks on the market, it's still (too) 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."

Baker went on to say that a sluggish back-to-school shopping season left a lot of inventory unsold, which hurt initial sales for Windows 8 devices.

Average selling prices of Windows computing devices have jumped significantly compared with last year, NPD said. Last year, overall prices were $433; this year's average selling price during the last four weeks was $477.

"The strong performance of Windows 8 notebooks with touch screens, where Windows 8 truly shines, offers some reason for optimism," Baker said. "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."

NPD said its research excluded sales of the Surface with Windows RT tablet, which also launched Oct. 26. Microsoft has not specified how many it has sold.

On Thursday, Microsoft announced the pricing for a premium version of that tablet.

Surface with Windows 8 Pro, which will be available in January, will cost $899 for a 64GB version and $999 for a 128GB version, the Redmond, Wash., company said.

The premium tablet looks much like the Surface with Windows RT model currently on the market. Both include dark titanium VaporMG casings, dual 2x2 MIMO antennas, and kickstands.

But the Surface Pro has Intel Corp.'s (INTC) next-generation Core i5 processor, which should give the tablet a graphics boost for its 10.6-inch display, which runs at a full-HD resolution. The Surface Pro also includes a full-size USB 3.0 port and a Surface pen with Palm Block technology. It will run current Windows 7 desktop applications.

"It's a full PC and a tablet," Panos Panay, general manager of Microsoft Surface, said in a blog post.

The device weighs less than 2 pounds and is less than 14 millimeters thick.