Microsoft has started sending letters to people who have already downloaded the company's Windows source code that was illicitly leaked last week, warning them that such actions are in violation of the law.

The Redmond, Wash.-based maker of the Windows operating system said it also has placed alerts on several peer-to-peer clients where such illegal sharing of the leaked source code has taken place. The alerts are designed to inform any user who conducts specific searches on these networks to locate and download the source code that such activity is illegal, the company said.

The letters come after portions of the company's Windows 2000 and Windows NT 4.0 source code appeared on the Internet last Thursday, Feb. 12th. Microsoft said the code leak was not the result of a breach in its corporate network or internal security. Neither did it lay the blame on participants in its Shared Source Initiative or Government Security programs. These initiatives provide access to the code for academic institutions and independent software vendors.

By press time, Microsoft executives did not return calls asking for clarification on how they identified rogue downloaders.

In a similar copyright-infringement battle waged by the Recording Industry of America, the RIAA subpoenaed customers' records from ISPs in order to sue infringing subscribers who download copyrighted songs. The RIAA also copied and used Kazaa's P2P software to find and warn people who downloaded protected material. (In January, Kazaa filed its own copyright infringement suit against the organization.) Kazaa officials did not answer requests for comment.

The RIAA and the Motion Picture Association America also seeded the Kazaa network with decoy copies of copyrighted files that gave downloaders a good nagging instead of the music they hoped for. For good measure, it sent warning instant messages to users who offered illegal recordings. Microsoft may employ similar guerilla tactics.

StreamCast hasn't been asked to work with Microsoft, according to Michael Weiss, CEO of the company, which makes the Morpheus file-sharing software application.

Wayne Grosso, CEO of Optisoft, creator of the Blubster, Piolet and MP2P Technology file-sharing applications, said Microsoft would have to sign onto each P2P network and offer dummy versions of the code, then hope users would download them instead of the real stuff, in order to put file-traders on notice. "Networks don't have a way to push something," Grosso asserted.

Microsoft said it working with the FBI to find the source of the leak. The company has promised to use its might to crack down on not only the original thief or thieves, but also on those who violate its copyright or trade secrets.