Microsoft will not have to pay $388 million to a security software company for infringing on a patent, U.S. District Judge William Smith ruled Tuesday.
Uniloc, which has headquarters in California, had accused Microsoft of infringing a patent related to the antipiracy software-registration system Microsoft uses as part of its product activation.
Microsoft's successful appeal comes after a jury found in April that the company violated Uniloc's patent, which Smith vacated Tuesday.
The case was heard in U.S. District Court for the District of Rhode Island.
The case, originally filed in September 2003, has taken several turns. Prior to the April decision, the same U.S. District Court found that no infringement had occurred.
But in August 2008, the U.S. Court of Appeals reversed that decision and sent the case back to the District Court for a re-trial, which ruled against Microsoft.
Microsoft's product-activation procedure links software to particular machines, which is intended to reduce piracy.
The applications can be reinstalled repeatedly on the same computer, but not installed on other ones.
Uniloc was not immediately available for comment.