Microsoft's decision to put two user interfaces (UIs) inside Windows 8 was a strategic mistake that fails novice and experienced computer users alike on both tablets and traditional PCs, a usability expert said Monday.

"That was the true strategic mistake, that they could do 'one Windows' on both tablets and PCs," said Jakob Nielsen, a usability expert with nearly 80 U.S. patents to his name. Nielsen, half of the consulting firm Nielsen Norman Group, was formerly a top-level engineer with Sun Microsystems, and has a Ph.D. in human-computer interaction.

"Windows 8 has low usability, amazingly low usability," Nielsen said in an interview Monday, ticking off a host of problems with the UIs of both the "Windows 8 Store" interface -- formerly "Modern," before that "Metro" -- and the "Classic" desktop that's a revamp of Windows 7.

In a Monday blog post, Nielsen cited issues such as the dual UIs, and the human "memory load" that results; a lack of multiple windows in the Windows 8 Store touch-oriented mode; hidden commands; low information density; and error-prone gestures.

Several weeks ago, Nielsen accused Microsoft of throwing users "under the bus" with Windows 8. Yesterday, he reasserted that Windows 8 is a UI mess, one that confuses and confounds users, even those familiar with Windows. And unlike many reviewers and Microsoft pundits, he contended that the Redmond, Wash. developer got it wrong even on the touch part of the operating system, which is virtually the only mode available on Windows RT, the operating system spin-off that powers tablets such as Microsoft's own Surface RT.

"Windows 8 on mobile devices and tablets is akin to Dr. Jekyll: a tortured soul hoping for redemption," Nielsen wrote in his blog. "On a regular PC, Windows 8 is Mr. Hyde: a monster that terrorizes poor office workers and strangles their productivity."

Nielsen based his take in part on testing he conducted with a dozen experienced PC users, who were asked to complete tasks on both traditional PCs and on the Surface RT.

"I had hope that the outcome was that Windows [RT] was great on the Surface RT," Nielsen said in the interview. "After all, that's what it was made for."

But testing revealed that users had serious and persistent problems figuring out what Windows RT wanted from them, and were challenged by relatively simple chores. In one test, the participants were asked to compile a list of three recommendations for a night out, send those suggestions to a friend and acknowledge the reply.

"That was enormously complicated on the Surface RT because of the single window model," said Nielsen, referring to the one-window-at-a-time UI for Windows RT apps. And even though the OS offers a split-screen mode, "None of our test users were able to make this work."

The single-window mode, chided Nielsen, should have required Microsoft to rename the operating system "Microsoft Window." But the problem lies in more than the nonsensical name.

"It's an extra burden on short-term memory," he said, referring to the need to open one app, remember what information was there, or even where that information was, then apply it to another app. "Short term memory is notoriously unreliable. Even something you'd do at home, maybe research a vacation, is difficult to do because it requires comparing and collecting information across multiple windows."