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.
More than a billion PCs currently run on Windows, including 670 million that use Windows 7, the last version of the operating system, released in 2009.
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.
Image: John Eversole, a marketing manager for Microsoft, uses a Samsung Series 7 Slate device with the new Windows 8 operating system.