Ballmer claims that "tens of millions" of corporate copies have shipped, but didn't specify install base.
Microsoft has revealed that 4 million users upgraded to Windows 8 over the opening weekend.
CEO Steve Ballmer confirmed the number during his keynote at the Build 2012 conference on Monday and predicted that 400 million devices will be running Windows 8 by the end of 2013. This would eclipse the uptake of Windows 7, which sold around 175 million copies in it's first year.
However, with many enterprises still making the jump from Windows XP to Windows 7, business uptake remains unclear. Ballmer was bullish in his assessment, claiming that "tens of millions" of corporate copies have been sent out to enterprises and that they will upgrade when they're ready.
Microsoft is working hard to ensure that Windows 8 is one of the most successful versions of the OS to date. The firm is spending an estimated $1bn on advertising and has closely tied in the desktop OS to the Windows Phone 8 mobile operating system.
Redmond is also encouraging users to upgrade to Windows 8 with a special offer. Any users who purchased a Windows 7 PC between 2 June 2012 and 31 January 2013 will be able to upgrade to Windows 8 Pro for £15.
It remains to be seen whether Microsoft will disclose sales of its Surface RT tablet, which enters a crowded market dominated by the iPad.