Windows 8 comes with new controls. It marks the first time Microsoft has made touch-screen control the top priority, though the system can still be navigated with a keyboard and mouse in desktop mode.
"In the case of Windows 8, seeing, touching, clicking and swiping is really believing," Ballmer said. He also predicted the PCs running on Windows 8 will be hailed as the best machines ever made.
Some Windows 8 PCs will be hybrids that look like laptops, but also have detachable display screens containing a separate battery so they can work like tablets, too. Those devices will face direct competition from Microsoft's Surface.
On Thursday, Microsoft also spent time touting the Surface as a more versatile and durable alternative to the iPad, still the most popular tablet on the market.
At one point, a Microsoft executive dropped the Surface on the stage floor to demonstrate how difficult it is to break. In another gimmick, another Microsoft executive stood on a Surface with wheels to show it even had the strength of a skateboard.
The Surface goes on sale Friday, priced at $499 for a Wi-Fi-only tablet with 32 gigabytes of storage. Apple charges the same price for its latest full-size iPad with half the storage capacity. The price for a separate Microsoft "touch cover" that also serves as an attachable keyboard starts at $120.
Apple rolled out its own artillery earlier this week when it showed off a series of improvements to its own laptop and desktop computers and debuted the iPad Mini, a smaller and less expensive take on its top-selling tablet. Google will return fire Monday in New York at an event that it expected to introduce yet another smartphone and a larger version of the company's 7-inch Nexus tablet.
Microsoft's decision to sell its own piece of Windows 8 hardware threatens to alienate the device makers who license its software at the same time many consumers could be expressing their dismay and confusion with the redesigned operating system.