Microsoft CEO Steve Ballmer celebrated the imminent release of the company’s Windows operating system after two years of work.

Windows 8 is the most dramatic overhaul of the personal computer market’s dominant operating system in 17 years.

Microsoft Corp. redesigned it so it will run on increasingly popular tablet computers, as well as PCs. Another version of Windows 8 is being released for smartphones next week.

PCs and tablets running on Windows 8 go on sale Friday. While appearing at an event in New York, Ballmer hailed Windows 8 PCs as the best machines ever made.

More than 1,000 PCs have been certified to run Windows 8. In a departure from its usual software-focused strategy, Microsoft is selling a tablet called Surface that runs on Windows 8.

“We kicked off a new era for Microsoft and a new era for our customers,” Ballmer said.

Windows 8 represents a big risk for Microsoft because Windows 8 looks and operates so much differently than previous versions.

The redesign discards the familiar “start” button and menu that Windows has had since 1995 when users are operating in a desktop mode, 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.

The new look comes with new controls. This 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.

The redesign could frustrate many long-time users who had grown accustomed to the old way of doing things and cause them to consider checking out computers made by Apple Inc., which already has been gaining market share in the past few years.

Microsoft’s decision to sell a Windows 8 tablet also threatens to alienate the device makers who license Windows 8 for their desktop PCs, notebook computers and tablets.

It’s a gamble that Microsoft felt it had to take because people are increasingly embracing smartphones and tablets such as Apple’s hot-selling iPad to take care of their computing needs. The shift has contributed to decline in PC sales, hurting Microsoft and the makers of desktop and laptop machines, including Hewlett-Packard Co. and Dell Inc.

—Copyright 2012 Associated Press