In papers filed with a federal appeals court, Bellevue, Wash.-based Zango argued that Kaspersky, which removes software that serves pop-up ads, should itself be considered "badware." "Kaspersky disabled Zango without the customer's consent and without the customer's ability to override Kaspersky's invasive actions," Zango argued to the Ninth Circuit in a brief filed late last week. "Kaspersky inflicted this interference by behaving like the very 'badware' from which it purported to protect consumers."

But Kaspersky's lawyer said this view isn't supported by the facts. "The reason consumers buy or download anti-spyware programs is precisely to have the option of addressing this particular type of software," said Bruce E.H. Johnson, attorney for Kaspersky, headquartered in Woburn, Mass.

Zango declined to elaborate on the arguments in its appellate brief.

Last August, federal district court judge John Coughenour in Seattle dismissed Zango's lawsuit on the grounds that the federal Communications Decency Act shielded Kaspersky from liability. That law has a "good samaritan" clause that protects interactive computer services from liability for good faith efforts to restrict the availability of material that users consider objectionable. While there have been a handful of other lawsuits against spyware removal companies, Zango's case against Kaspersky appears to mark the first time a court ruled definitively on the issue.

Eric Goldman, director of the High Tech Law Institute at Santa Clara University School of Law, said that if Coughenour's interpretation of the statute holds sway with appellate courts, adware companies are likely to stop bringing lawsuits against companies that sell removal software. "The beauty of a clean interpretation," he said, "is it eliminates the threat of litigation between adware vendors and anti-spyware vendors."

The watchdog organization Center for Democracy & Technology, which two years ago complained about Zango to the Federal Trade Commission, is considering asking to file a friend-of-the-court brief in the case, said Ari Schwartz, deputy director of the group. The organization had alleged to the FTC that Zango affiliates installed the company's ad-serving software without first obtaining consumers' valid consent.

The FTC in 2006 brought its own complaint against Zango, and the adware company agreed to pay a $3 million fine, refrain from installing adware without first making sure that people consented, and to monitor third-party distributors to make sure they aren't installing the software without consent.

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