The introduction of the Windows 8 operating system hasn\'t lifted U.S. sales of PCs or tablet computers, sales and marketing analysis firm NPD Group says. Windows device sales have fallen 21 percent compared to the same period last year following the highly anticipated launch of the software Oct. 26, the market research firm said. Notebook sales were down 24 percent, desktop sales dropped 9 percent and Windows 8 tablet sales have \"been almost nonexistent,\" 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,\" Stephen Baker, vice president of industry analysis at NPD, told the Los Angeles Times. \"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.\" The sales figures excluded Microsoft\'s Surface tablet with Windows RT which also launched Oct. 26, NPD said. Microsoft has not specified how many of the tablets it has sold.