San Francisco - Microsoft said it sold 40 million licences for its Windows 8 operating system in the first month after a launch which got a mixed reception.One of the new heads of the Windows unit, Tami Reller, made the announcement to an analysts' meeting and Microsoft provided the information on its Windows blog."Windows 8 is outpacing Windows 7 in terms of upgrades," the blog post said. "We built Windows 8 to work great on existing Windows 7 PCs. And we also set out to make upgrading from Windows 7 to Windows 8 super easy."Microsoft released the new operating system worldwide on 26 October as a way to help the dominant PC software maker get a bigger share of the market for mobile devices such as tablets.But some say the new system may be hard to adapt to both PC and tablet format and that businesses may be slow to adopt Windows 8.It's not clear how many people are using Windows 8, because the licences may be sold to PC makers and not yet in use.Windows head leaving MicrosoftEarlier this month, Microsoft said the head of its Windows unit was leaving.Windows president Steven Sinofsky will depart Microsoft and, effective immediately, his duties will be divided between a pair of executives who will answer directly to chief executive Steven Ballmer.Julie Larson-Green was promoted to lead Windows software and hardware engineering. Reller will run the business side of Windows in addition to her duties as chief financial officer.Microsoft has not disclosed sales data for its new Surface tablet computer, which uses Windows 8, and was launched at the same time as the operating system.The Redmond, Washington giant said it was also boosting the number of apps in its Windows Store as part of its mobile strategy."There were more apps in the Windows Store at launch than any other app store at their launch and since then, the number of apps in the Windows Store has doubled," the blog said."A number of apps in the Windows Store have crossed the $25 000 revenue mark and the developer keeps 80% of the revenue they make off downloads for the life of their app. A lot of great new apps have been added to the Windows Store since launch."