Sales of personal computers in the U.S. didn’t get any boost from the launch of Windows 8, confounding the hopes of Microsoft and PC makers, research firm NPD Group said Thursday.

In the three weeks after Windows 8 launched on Oct. 26, sales of Windows PCs and tablet computers fell 21 percent compared with a year earlier, according to NPD, which tracks U.S. retail sales.

There’s no sign that Windows 8 made things worse for PC makers. Rather, NPD said the weak sales are a continuation of a trend seen throughout this year.

U.S. consumers appear more interested in spending electronics dollars on smartphones and tablets than on upgrading their computers. Several tablet makers have released new models this fall, and Apple (Nasdaq: AAPL) has made a smaller version of its market-leading iPad.

Microsoft Corp. (Nasdaq: MSFT) developed Windows 8 with tablets and touch screens in mind. The new operating system features a new look and is designed to make desktops and laptops work more like tablets.

The Redmond, Wash., company is supporting the launch with its largest-ever advertising campaign. UBS analyst Mary Jo Foley put the value of the campaign at $1 billion.

Reviews Mixed

PC makers were officially optimistic and have largely switched their consumer-focused product lines over to Windows 8, but they were cautious about stocking large numbers of PCs ahead of the launch. Reviews of the new operating system were mixed.

“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,” NPD analyst Stephen Baker 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.”

Baker said Windows 8 laptops with touch screens were doing well, helping drive up the average selling price of PCs.

Microsoft has said it sold 40 million licenses to Windows 8 in its first month on the market, but that number includes licenses bought by PC manufacturers such as Hewlett-Packard Co. and Dell Inc. for machines built but not yet sold.

Research firm IHS iSuppli has predicted that 349 million PCs will be shipped worldwide this year. That would be a 1 percent decline from the year before — the first year-over-year decline since 2001. In the U.S., PC sales have been declining for two years already.

Surface Pro Tablet Prices Set

Also Thursday, Microsoft said its Surface tablet computer running the new Windows 8 Pro software will go on sale in January, the first versions of the machine that run on Intel Corp. chips.

Surface tablets with Windows 8 Pro will cost $899 for a version with 64 gigabytes of memory, and $999 for a 128-gigabyte model. They run on Intel’s Core i5 processor, giving the machines a graphics boost.

Microsoft hasn’t disclosed how many units it has sold of the Surface that runs on a chip with ARM Holdings Plc technology, which went on sale last month for $499 to $699. The price for the Intel-based iteration, which doesn’t include the tablet cover with a built-in keyboard that is one of Surface’s key features, may be too high for many users.

“At that price, by the time you add the keyboard you’re at the price of an 11-inch MacBook,” said Wes Miller, an analyst at Kirkland, Washington-based Directions on Microsoft, referring to Apple Inc.’s laptop computer. “Everyone always says, ‘well, Apple is so expensive,’ but here we are in that same neighborhood looking at houses.”

Still, Miller said the price for the Surface with Windows 8 Pro is more reasonable if a user is planning to have the machine double as a tablet and a full personal computer. For example, customers could put the machine in a dock and use it alongside a larger screen to replace desktop computers, he said.

“But if you’re looking at it as just a tablet, it is pricey,” he said.

(The AP and Bloomberg contributed to this report.)