Windows 8's reception was something of a mixed bag with plenty of opinions in favor and against the new touch-centric interface that takes center stage over the classic Windows desktop. But that isn't stopping consumer adoption of the new OS, apparently. Speaking during his keynote address at the Build 2012 conference, CEO Steve Ballmer told attendees they've sold 4 million upgrades already.

That figure is understood to include individual purchases only as Ballmer also mentioned that “tens of millions” of Windows 8 upgrade units have been sold to corporate customers. According to the chief executive, the “level of interest and enthusiasm” for Windows 8 and the Surface has been “stunning”, although he didn't share any numbers for the latter.

Ballmer is hoping to convice more developers to create applications for its new operating system, as well as the Windows Phone 8 OS launched yesterday. 

So far, companies like Netflix, Hulu, Evernote and eBay have already contributed apps. Other upcoming apps from high-profile developers are being showcased at the event, including clients for Dropbox and Twitter, as well as a PayPal API that will allow developers to use the payment system within any Windows Store app.

Ballmer says that a year from now we’ll see close to 400 million new devices running Windows 8, making it "the single largest opportunity for software developers today." That's certainly a bold prediction considering Windows 7 has sold close to 700 million copies since its release in 2009. Microsoft is hoping that its entry into the tablet market will help bolster sales of its operating system cash cow even further this time around.