Microsoft spokesman Jack Evans said in an e-mail, "The Ninth Circuit's decision not to accept our request for interim review is not a ruling on the merits of our case. We look forward to the ultimate dismissal of what the district court itself said is a novel claim."

Discovery in this case has been damaging to Microsoft. E-mails that came out in the class certification process outlined an internal debate about Vista's marketing and readiness for certain types of hardware.

The case stems from a Microsoft program to prop up PC demand after the release of Windows Vista was delayed, missing the 2006 holidays -- a key sales period for consumer PCs.

The plaintiffs alleged that PCs marked "Windows Vista Capable" could run only the "Basic" version of the operating system, not the "Premium" version, which they consider to be the "real" Vista, based on the company's marketing. Microsoft argued that its marketing delineated between machines that could run the Premium version and those that could not.

In February, U.S. District Court Judge Marsha Pechman certified a class of people who purchased "Windows Vista Capable" PCs that did not also bear the "Premium Ready" designation.