Microsoft filed 26 federal lawsuits against companies suspected of dealing in pirated copies of its lucrative Windows and Office computer software, a release said.
Microsoft gathered evidence against software sellers and installers through an investigation that involved "secret shoppers" posing as customers, according to the Redmond, Washington, software powerhouse.
The lawsuits were filed in the US states of Colorado, Georgia, Illinois, Ohio, New Jersey, New York, and South Carolina, Microsoft said.
The move was part of a Genuine Software Initiative that included automatically checking the legitimacy of Microsoft software on people's computers as a condition of getting program updates.
The Business Software Alliance reportedly estimated that 21 percent of all software in the United States is pirated.
"We are committed to finding the unscrupulous dealers of pirated software and making piracy a business model that doesnt work," Microsoft senior attorney Mary Jo Schrade said in a release."