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.

Some have criticized Mission Control, Apple’s new centralized app and window management interface, saying that it adds complexity rather than introducing the simplicity of a mobile interface. At the other extreme, Lion allows any app to be rendered full-screen, which blocks out distractions but also forces users to switch applications more often than necessary.

“The problem [with a desktop OS] is that it’s hard to manage windows,” says Mathis. “The solution isn’t to just remove windows altogether; the solution is to fix window management so it’s easier to use, but still allows you to, say, write an essay in one window, but at the same time look at a source for your essay in a different window.”

Windows 8, meanwhile, attempts to solve this problem in a more elegant way, with a “Windows Snap,” which allows apps to be viewed side-by-side while eliminating the need to manage their dimensions by dragging them from the corner.

A problem with moving toward a touch-centric interface is that the mouse is absolutely necessary for certain professional applications. “I can’t imagine touch in Microsoft Excel,” says Shive. “That’s going to be terrible,” she says.

The most significant difference between Apple’s approach and Microsoft’s is that Windows 8 will be the same OS no matter what device it’s on, from a mobile phone to a desktop PC. To accommodate a range of devices, Microsoft has left intact the original Windows interface, which users can switch to from the full-screen start screen and full-screen apps option.

Merholz believes Microsoft’s attempt to make its interface consistent across all devices may be a mistake. “Microsoft has a history of overemphasizing the value of ‘Windows everywhere.’ There’s a fear they haven’t learned appropriateness, given the device and its context,” he says.

Shive believes the same could be said of Apple. “Apple has been seduced by their own success, and they’re jumping to translate that over to the desktop … They think there’s some kind of shortcut, where everyone is loving this interface on the mobile device, so they will love it on their desktop as well,” she says.

In a sense, both Apple and Microsoft are about to embark on a beta test of what the PC should be like in an era when consumers are increasingly accustomed to post-PC modes of interaction. But it could be a bumpy process. “I think we can get there, but we’ve been using the desktop interface for 30 years now, and it’s not going to happen overnight,” says Shive.