Microsoft is objecting to a plan that would force the company to use its Windows Update service to notify potential members in the "Windows Vista Capable" class-action lawsuit, according to documents filed in federal court Wednesday.

In a Wednesday filing, Microsoft called the idea, which was put forward two weeks ago by plaintiffs' lawyers, an "attempt to hijack Windows Update" that would bombard millions with a message that "amounts to spam."

The Vista Capable lawsuit accuses Microsoft of duping buyers in 2006 and 2007 by letting PC makers slap a "Vista Capable" sticker on PCs when it allegedly knew that many of those systems could run only Vista Home Basic, the entry-level version. The case, which began in 2007 and was granted class-action status in February 2008, claims that Home Basic is not representative of the Vista that Microsoft marketed to consumers.

Earlier this month, the plaintiffs in the Vista Capable case asked US District Court Judge Marsha Pechman to make Microsoft use the update service to send all Windows users a notice of the class-action lawsuit. The notice, which would pop-up on users' screens, would include a link to a site where consumers could obtain more information.

Windows Update is best known for delivering security patches on the second Tuesday of each month, although it has also been used by Microsoft to push non-security updates and to patch third-party products. It has not been used for legal messages such as the one proposed by the plaintiffs' lawyers, however.

"The Court should deny Plaintiffs' attempt to hijack Microsoft's Windows Update service to distribute class notice," the objection filed today read. "Microsoft has told consumers and businesses that it uses Windows Update for software updates to the Windows operating system, never for general informational messages. Plaintiffs' plan, however, would use Windows Update to foist irrelevant notice on persons owning over 120 million PCs that are not the subject of this case, who should not be forced to spend time reading a notice that, as to them, amounts to spam."

Microsoft also argued that users would revolt if Windows Update was used for something other than updates. "Distribution of notice via Windows Update would likely cause some users to be upset that Microsoft acted contrary to their expectations," said the company's objection, "and they may go so far as to turn off Windows Update and lose the important protections that it provides."

The company has already faced users angry about how Microsoft uses Windows Update. A 2006 lawsuit, for example, accused Microsoft of misleading users when it delivered an update for its Windows Genuine Advantage (WGA) anti-piracy software via the service. That suit, which has sought class-action status, is ongoing. Microsoft, in fact, acknowledged the earlier lawsuit in its filing Wednesday.

"Microsoft never has used Windows Update to distribute promotions, advertisements, news, or other general information to users computers," the company claimed Wednesday. "Nor has Microsoft ever been ordered to use Windows Update to distribute class notice."