The effort began weeks ago, after a hacker released a program dubbed FairUse4WM that strips digital rights protection from media files distributed by companies like Movielink and Amazon.com Inc., which launched a video downloading service last week.

The Redmond-based software maker released its first fix on Aug. 28. The hacker quickly released another version of the program, promoting it on Internet message boards as a way to allow "fair use" of purchased digital media files.

Marcus Matthias, a senior product manager in the Microsoft division that includes Windows, said the company is "actively working on an update" and keeping digital content providers informed about its progress.

Matthias had no estimate of how long it would take Microsoft to come up with its next update.

The "digital rights management" tool in Windows Media software is designed to prevent Internet users from freely copying movies, songs or other digital files, but analysts say there's virtually no way to make it fully hack-proof.

"Any protection that's just software can always be broken by other software. Some attacker was bound to be able to do something like this," said John Pescatore, vice president of Internet security at the market research firm Gartner Inc.

One company that relies on Media Player to distribute content, British satellite broadcaster BSkyB, temporarily suspended its film digital download service on Aug. 31, citing Microsoft's efforts to update software protecting against unauthorized copying of files.

Amazon.com spokesman Sean Sundwall said the Internet retailer knows about the problem, but hasn't experienced any related troubles with its new Unbox service, which lets people watch TV shows or movies on Windows Media-compatible portable devices.

Marc Maiffret, co-founder and chief hacking officer at eEye Digital Security, said he doubts this will be the last time Microsoft has to contend with this type of problem.

"I think it's one of those things where they have to show they're trying to do something about it," Maiffret said. "Hopefully, they're not so dumb that they think they can put a dent in music piracy."