Microsoft is struggling to capture the imagination of Americans, with the company behind on Windows 8 sales across the country.

According to US retail sales monitor NPD Group, Windows 8 has failed to reignite any passion for PCs in America, with sales down 21% compared to the same period last year.

Speaking to English newspaper The Guardian, the group also revealed the new operating system is significantly lagging behind previous version Windows 7.

NPD says Windows 7 figured in 83% of new device sales during 2009, compared with 58% for Microsoft's latest release.

Explaining the weakness in PC sales due to the rise of tablets, NPD believes the slow sales implies that PC vendors and retailers still have a large amount of inventory with Windows 7.

With any backlog needing to be cleared before Windows 8 can properly sell, the group also credits a slow "back to school" period during September and October as another blow for the software giant.

Usually defined as a period of strong sales, NPD's vice president Stephen Baker thinks this could be one of the key reasons for the slow sales.

"The bad back to school period 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 news comes less than three days after Microsoft sold 40 million Windows 8 licences since its launch last month.

Despite concerns across America, the company said sales of the operating system eclipsed that of Windows 7 three years ago, representing a solid start for the product.

While the Microsoft did not offer an exact comparison, finance and marketing head of Windows Tami Reller suggested sales were ahead of the previous system, which shifted 60 million units during the first ten weeks of sale in 2009.

“Windows 8 upgrade momentum is outpacing that of Windows 7,” Reller said.