Ballmer said Windows 8 will also have a range of tools, including built-in search, news, finance, sports and other apps powered by Bing, Microsoft's search engine.
Bing has been struggling to compete with Google's search engine. Having it prominent in Windows 8 could help boost traffic to it.
Ballmer appeared on stage.
"Windows 8 shatters perceptions of what a PC now really is," he said. "We've truly reimagined Windows and kicked off a new era of Microsoft and for our customers."
Microsoft showed off five devices running Windows RT, a lightweight version of Windows 8. Unlike the full version, RT can run only apps developed for it and obtained from the Windows online store.
One of them was the Surface. Other machines shown included ones from Dell Inc., Samsung Electronics Co. and AsusTek Computer Inc.
The event was being held in a cavernous former bus depot, with more than 500 people in the audience. Pier 57 is located on the Hudson River, near the city's Meatpacking District.
There have been few surprises, as Microsoft announced the release date and made preview versions available long ago. Microsoft's stock climbed 3 cents to $27.93 in midday trading. Trading volume was low, less than half of the average.