Microsoft has sent threatening legal letters to Internet users that have downloaded the Windows source code that was leaked online last week and is warning others looking for it not to download it.

The software giant claims to be sending warnings to people who search for the code on file-swapping networks and mailing legal warnings to those who have already downloaded the secret programming code.

However, Microsoft spokesman Tom Pilla would not say which P-to-P services Microsoft is targeting or how the company is delivering the alerts on services it doesn't own. "These alerts serve to inform users who initiate a search looking for Windows source code that it is illegal to view, download and share the code," he said.

Microsoft has also sent out hundreds of letters to people who have already downloaded the source code, Pilla said. Many have already responded by agreeing not to distribute the code and deleting it from their systems, he claimed, but would not detail how Microsoft knows who downloaded the code or who received the letters.

"We will take all appropriate legal actions to protect our intellectual property. Today is about communicating, notifying and explaining to users that downloading, sharing and viewing of the source code is illegal," Pilla said.

Microsoft last week said that incomplete portions of its closely-guarded Windows NT 4.0 and Windows 2000 source code had been leaked on the Internet. The company has started an internal investigation and called in the FBI.

Pursuing those who have downloaded the code and warning Internet users not to download the code is a "natural move" from Microsoft, said Thor Larholm, a senior security researcher at PivX Solutions LLC in Newport Beach, California, adding that he has not downloaded the code himself.

"Microsoft has historically gone to great lengths to defend their copyright, intellectual property and trade secrets," he said. "Companies developing products for the Windows platform should also ensure that their employees do not have the leaked source code, as this could impact on their operations and bring a significant liability on the company."

Although it is within Microsoft's rights to protect its intellectual property, playing hardball with those who have downloaded it may have an adverse effect on the company's image, warned Joe Wilcox, a Jupiter Research senior analyst in Washington DC.

"While the company arguably is the wronged party here, Microsoft may want to carefully consider the larger public image cost of trying to protect three-and-a-half year-old source code," he said. "The notices run the risk of backlash at a time when Microsoft tries to rebuild a public image tarnished by the US anti-trust case."

Wilcox said that some people may view Microsoft's tactic as heavy-handed, along the lines of Recording Industry Association of America. "Remember that some of the source code trading occurs on the same P-to-P networks used for swapping music," he said.

Microsoft's Pilla would not comment on what further steps Microsoft may take as it works to protect its intellectual property and trade secrets.