Microsoft CEO Steve Ballmer is celebrating the imminent release of the company's Windows operating system after two years of work.

Pedestrians pass the soon-to-be-unveiled Microsoft store in Times Square (John Moore/Getty Images)

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.

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