Microsoft has filed 21 lawsuits in 14 U.S. states against resellers in which it alleges they engaged in the sale of pirated software, it said Tuesday.

Some of the cases were caught thanks to Microsoft's Windows Genuine Advantage or Office Genuine Advantage programs in which the copy of the Windows operating system or Office suite on a user's computer is electronically authenticated online. Microsoft began rolling out Windows Genuine Advantage in April 2006.

Others lawsuits were sparked by calls to Microsoft's anti-piracy hotline, the company said.

Almost all the lawsuits all concerned Windows XP and/or consumer versions of Office. Only two suits alleged piracy of an enterprise version of the software and Windows Vista was not the subject of an allegation in any filing, according to information from Microsoft.

Among the companies against which lawsuits were filed were several that had been previously been accused by Microsoft of software piracy and had settled with the company, it said.

The lawsuits were filed in California, Georgia, Massachusetts, Ohio, South Carolina, Tennessee, Texas, Virginia, Oregon, Arkansas, Colorado, Florida, Illinois and Washington.

According to a recent report published by the Business Software Alliance, an anti-piracy group funded by software makers, the U.S. had a piracy rate of 20 percent -- the lowest in the survey. Although due to the size of the market losses from piracy in the U.S. were estimated at $US8 billion, which is higher than any other single country.