NEW YORK – The release of Microsoft’s Windows 8 operating system is a week away, and consumers are in for a shock. Windows, used in one form or another for a generation, is getting a completely different look that will force users to learn new ways to get things done.

Microsoft is making a radical break with the past to stay relevant in a world where smartphones and tablets have eroded the three-decade dominance of the personal computer. Windows 8 is supposed to tie together Microsoft’s PC, tablet and phone software with one look. But judging by the reactions of those who have tried the PC version, it’s a move that risks confusing and alienating customers.

Tony Roos, an American missionary in Paris, installed a free preview version of Windows 8 on his aging laptop to see whether Microsoft’s new OS would make the PC faster and more responsive. It didn’t, he said, and he quickly learned that working with the new version requires tossing out a lot of what he knows about Windows.

“It was very difficult to get used to,” he said. “I have an 8-year-old and a 10-year-old, and they never got used to it. They were like, ‘We’re just going to use Mom’s computer.’ ”

Windows 8 is the biggest revision of Microsoft Corp.’s operating system since it introduced Windows 95 amid great fanfare 17 years ago. Ultimately, Windows grew into a $14 billion-a-year business and helped make Bill Gates the richest man in the world for a time. Now, because of smartphones and tablets, the PC industry is slumping. Computer companies are desperate for something that will get sales growing again. PC sales are expected to shrink this year for the first time since 2001, according to research firm IHS iSuppli.

The question is whether the new version, which can be run on tablets and smartphones, along with the traditional PC, can satisfy the needs of both types of users.

“I am very worried that Microsoft may be about to shoot itself in the foot spectacularly,” said Michael Mace, the CEO of Silicon Valley software startup Cera Technology. Windows 8 is so different, he said, that many Windows users who aren’t technophiles will feel lost, he said.

Microsoft is releasing Windows 8 next Friday, and it doesn’t plan to cushion the effect. Computer companies will make Windows 8 standard on practically all PCs.

“In the quest for simplicity, they sacrificed obviousness,” said Sebastiaan de With, an interface designer and the chief creative officer at app developer DoubleTwist.

Speaking to Wall Street analysts this week, Microsoft’s chief financial officer Peter Klein said he isn’t concerned that user confusion could slow the adoption of Windows 8.