Another version of Windows 8 will be released next week for smartphones, which are overwhelmingly dominated by Apple Inc. and Google Inc.'s Android software.
Microsoft is also opening a Windows 8 store featuring applications built to run on the system. The store is similar to the apps stores of Apple and Google and will include many of the same services.
About 670 million PCs currently run on Windows. But the owners of most existing Windows machines aren't expected to switch to Windows 8 for at least a year, maybe longer. That means most of Windows 8's early usage will come from consumers, businesses and government agencies that buy new devices with Windows 8 already installed.
There were few surprises because Microsoft Corp. has been previewing preliminary versions of Windows 8 for the past 13 months — part of 1 billion hours of testing.
Still, the overhaul poses a big risk for the Redmond, Wash., company because Windows 8 looks and operates so much differently than previous versions.
"This is the biggest gamble they've ever made," said analyst Richard Doherty of the Envisioneering Group. "Does (Windows 8) do more things? Yes ... but it's not that easy to use."
Even when users revert to a desktop mode, the redesign discards the familiar "start" button and menu that Windows has had for 17 years, a change that critics believe will almost certainly provoke howls of protest. But many reviewers applaud Microsoft for overhauling Windows so it greets users with a mosaic of tiles displaying applications instead of relying on the desktop icons that served as the welcome mat for years.
In a brief interview Thursday, Ballmer expressed confidence that PC users would quickly realize the mosaic is easier and quicker to use than the old desktop format.
"You've got a whole screen as a start button!" he said.