The keynote has ended, although the launch event continues. Microsoft plans to return at 1:30 p.m. with a presentation on Surface.
Ballmer said outside developers have been working "fast and furiously" to make apps for Windows. He said developers have praised how they can write the same app for a variety of devices — desktops, laptops and tablets.
"Windows 8 PCs really are the best PCs ever," he concluded.
He encouraged people to visit a store to try Windows 8 themselves.
"Seeing, touching, clicking and swiping really is believing," he said.
Ballmer talked about the phone version of Windows, known as Windows Phone 8. There's an event planned Monday.
"I can't wait to show you how we've really reinvented the smartphone around you," he said.
In telling people that their relationship with machines will change, Ballmer said, "Picture your start screen filled with everyone and everybody who's most important to you."
He added, "Windows 8 will help you do everything and will make it a lot of fun, frankly, to do ... nothing."
He said people will have several devices to choose from, including new designs that combine the best of PCs and tablets.