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Tami Reller, finance and marketing head of the Windows business, said "Windows 8 upgrade momentum is outpacing that of Windows 7."
The company sold 60 million licenses of Windows 7 in two months. Sales of Windows 7 were started in October 2009.
The latest figure does not mean that 40 million users have adopted Windows 8, Reuters reported.
Many of the sales are to PC manufacturers, who in turn sell a large number of machines to companies, very few of which are using Windows 8 yet.