How misleading was Microsoft's "Windows Vista Capable" campaign? Misleading enough for a judge to approve a federal trial. On Tuesday, a federal judge ruled that two PC buyers, Dianne Kelley and Kenneth Hansen, can proceed with a lawsuit that accuses Microsoft of deliberately deceiving the public.

The basis of the deception comes from the "Window Vista Capable" stickers it slapped on Windows XP machines in the run-up to Vista's January debut, InformationWeek reports. Microsoft had asked for a dismissal, but US District Judge Marsha Pechman of Seattle is happy to oversee an October trial.

With their class action suit, filed in March, Kelley and Hansen claim that Microsoft used "bait and switch" tactics to move XP systems as the world waited for the long-delayed Windows Vista. They argue that Microsoft's stickers reassured buyers "that they were purchasing Vista-capable machines when, in fact, they could obtain only a stripped down operating system." Some "Vista Capable" machines, they say, were only capable of running versions of the OS that lack things like Microsoft's new Aero interface and a spruced-up Windows Media Center.

Indeed, Microsoft offers a neutered version of Vista, known as Home Basic, that requires nothing more than an 800-MHz processor, 512MB of memory, a 20GB hard drive, and support for Super VGA graphics. And though the company recommends a 1GHz processor, 1GB of memory, a 40GB hard drive, and DirectX 9 support for "premium" versions of the operating system, these editions will still run on less powerful machines - with certain OS features disabled.

Microsoft has argued that its "Vista Capable" marketing campaign also included a "Vista Premium Ready" component that explained the differences between various versions of the OS, but Judge Pechman believes there's reason enough to let the suit go ahead.

Ray Persons, a partner with the international law firm King & Spalding who specializes in class actions, is amazed that Pechman approved a trial - let alone that she approved it so quickly. "It's awfully unusual for a court to certify a class action where the allegation is a bait and switch," he told The Reg. "In order to prove fraud, you've got to look at whether the individual reasonably relied on the defendant's alleged misrepresentations, and that kind of individual-by-individual determination really defeats the efficiencies of a class action."

Of course, it isn't likely that the trial will actually happen. "In most instances, civil cases - including class actions - get settled," Persons said. "Seldom, if ever, is a class action tried."

PC buyers who believe they've been duped by Microsoft can expect low-value coupons towards the purchase of machines that run premium versions of Windows Vista. These might fetch a few dollars on eBay.