Over the years, Keith McCarthy has become used to a certain way of doing things on his personal computers, which, like most others on the planet, have long run on Microsoft's Windows software.
But last week, when he got his hands on a laptop running the newest version of Windows for the first time, he was flummoxed.
Many of the familiar signposts from PCs of yore are gone in Microsoft's new software, Windows 8 - released on Thursday - like the Start button for getting to programmes and the drop-down menus that list their functions.
It took him several minutes just to figure out how to compose an e-mail message in Windows 8, which has a stripped-down look.
"It made me feel like the biggest amateur computer user ever," said McCarthy, 59, a copywriter in New York.
Windows has got a radical makeover, a rare move for a product with such vast reach. The new design is likely to cause some head-scratching for those who buy the latest machines when Windows 8 goes on sale Friday.
To Microsoft and early fans of Windows 8, the software is a fresh, bold reinvention of the operating system for an era of touch-screen devices like the iPad, which are reshaping computing. Microsoft needs the software to succeed so it can restore some of its fading relevance after years of watching Apple and Google outflank it in the mobile market.
To its detractors, though, Windows 8 is a renovation gone wrong, one that will needlessly force people to relearn how they use a device every bit as common as a microwave oven.
"I don't think any user was asking for that," said John Ludwig, a former Microsoft executive who worked on Windows and is now a venture capitalist in the Seattle area. "They just want the current user interface, but better."
Jakob Nielsen, a user interface expert at the Nielsen Norman Group, said Windows 8 was more suitable for tablet computers with their smaller displays, but it was not helpful for workers who needed to have lots of applications visible at once.