Apple is widely expected to unveil a major update this week to OS X Lion, its operating system for desktop and laptop computers. Microsoft, meanwhile, is working on an even bigger overhaul of Windows, with a version called Windows 8.

Both new operating systems reflect a tectonic shift in personal computing. They incorporate elements from mobile operating systems alongside more conventional desktop features. But demos of both operating systems suggest that users could face a confusing mishmash of design ideas and interaction methods.

Windows 8 and OS X Lion include elements such as touch interaction and full-screen apps that will facilitate the kind of "unitasking" (as opposed to multitasking) that users have become accustomed to on mobile devices and tablets.

"The rise of the tablets, or at least the iPad, has suggested that there is a latent, unmet need for a new form of computing," says Peter Merholz, president of the user-experience and design firm Adaptive Path. However, he adds, "moving PCs in a tablet direction isn't necessarily sensible."

Cathy Shive, an independent software developer, would agree. She developed software for Mac desktop applications for six years before she switched and began developing for iOS (Apple's operating system for the iPhone and iPad). "When I first saw Steve Jobs's demo of Lion, I was really surprised—I was appalled, actually," she says.

Shive is surprised by the direction both Apple and Microsoft are taking. One fundamental dictate of usability design is that an interface should be tailored to the specific context—and hardware—in which it lives. A desktop PC is not the same thing as a tablet or a mobile device, yet in that initial demo, "It seemed like what [Jobs] was showing us was a giant iPad," says Shive.

A subsequent demonstration of Windows 8 by Microsoft vice president Julie Larson-Green confirmed that Redmond was also moving toward touch as a dominant interaction mechanism. One of the devices used in that demonstration, a "media tablet" from Taiwan-based ASUS, resembled an LCD monitor with no keyboard.

Not everyone is so skeptical about Apple and Microsoft's plans. Lukas Mathis, a programmer and usability expert, thinks that, on balance, this shift is a good thing. "If you watch casual PC users interact with their computers, you'll quickly notice that the mouse is a lot harder to use than we think," he says. "I'm glad to see finger-friendly, large user interface elements from phones and tablets make their way into desktop operating systems. This change was desperately needed, and I was very happy to see it."

Mathis argues that experienced PC users don't realize how crowded with "small buttons, unclear icons, and tiny text labels" typical desktop operating systems are.

Lion and Windows 8 solve these problems in slightly different ways. In Lion, file management is moving toward an iPhone/iPad-style model, where users launch applications from a "Launchpad," and their files are accessible from within those applications. In Windows 8, files, along with applications, bookmarks, and just about anything else, can be made accessible from a customizable start screen.