Aug. 7 (Bloomberg) -- Microsoft Corp., the world's biggest software maker, must face claims that it infringes a patent on a registration system to prevent pirated copies, an appeals court ruled today.

The U.S. Court of Appeals for the Federal Circuit in Washington revived the suit by Uniloc Singapore Private Ltd. and Irvine, California-based Uniloc USA Inc., saying there was a ``genuine issue of material fact'' as to whether Microsoft uses the algorithm and steps covered by the patent. The appeals court overturned a lower-court ruling in Microsoft's favor and sent the case back for further review.

The 2003 dispute is over a patented software-registration system that requires unique identification for software licensees. Microsoft, based in Redmond, Washington, had argued that its product-activation feature was different than that covered by the Uniloc patent.

``Microsoft's defenses against Uniloc's claims remain strong,'' Microsoft spokesman Jack Evans said in an e-mailed statement. ``We continue to maintain that the Uniloc patent is invalid and not infringed by Microsoft technology.''

Uniloc spokesman Joe Austin said the company is reviewing the opinion.

Last week, the Federal Circuit revived a patent suit against Microsoft over a process to improve images on computer screens.

Microsoft rose 37 cents to $27.39 at 4 p.m. New York time in Nasdaq Stock Market trading.

The case is Uniloc USA Inc. v. Microsoft Corp., 2008-1121, U.S. Court of Appeals for the Federal Circuit. The lower court case is Uniloc USA Inc. v. Microsoft Corp., 03cv440, U.S. District Court for the District of Rhode Island (Providence).

To contact the reporter on this story: Susan Decker in Washington at sdecker1@bloomberg.net.

Last Updated: August 7, 2008 16:50 EDT