Microsoft's finance and marketing head of the Windows business, Tami Reller, has told an investors conference that the company has already sold 40 million licences for its Windows 8.

The latest version of its flagship operating system was only released at the end of October but is already outpacing its predecessor, Windows 7, which sold just over 60 million units in the first 10 weeks on sale at the end of 2009.

"The journey is just beginning, but I am pleased to announce today that we have sold 40 million Windows 8 licenses so far," said Reller, adding that the operating system shaping up to be one of the company's most successful products.

Previous versions of Windows sold considerably fewer licenses in a comparable period, with XP shipping eight million units in the month following its launch in 2002, and Vista selling around 10 million in its first month in 2006.

However, there are many more PCs in the world today and significant proportion of Windows 8 sales are to PC manufacturers, who in turn sell a large number of machines to companies.

The operating system is also priced far lower than previous versions, with an upgrade to Windows 8 costing $40, compared to $70 for the full software.

Windows 8 is the most radical overhaul of the operating system in years as Microsoft looks to compete in a new era where smartphones and tablets dominate consumer electronics.

It's designed to be used as both a touch screen interface as well as a traditional desktop operating system.

Reller also pointed to a number of other statistics indicating how well customers are adapting to the new user interface.

"When people experience Windows 8, they do find it is easy to get started and fun to learn," she said. "We know from the data we're getting in that customers do indeed get the product."

She said that using remote telemetry, the company has logged over 1.5 billion impressions of users deploying the new start screen, which means people are using the operating system just as Microsoft intended.

Reller recently took up her new position following the departure of Steven Sinofsky, who was the president of the Windows division for three years until he stepped down earlier this month. She joined Microsoft in 2001 as part of the acquisition of Great Plains Software, where she served as CFO.