Microsoft is taking steps to stop its leaked Windows source code spreading widely on the internet.
Gates: Keen to stop the code being shared
It is using the messaging systems in file-swapping networks to alert people sharing the code that what they are doing is illegal.
Microsoft has also written to those letting others download the code from them that such sharing is illegal.
It has said it will take "appropriate" steps to protect its software but has yet to launch any legal action.
Stopping the copying
Since source code for Microsoft Windows NT 4.0 and 2000 was found online on 13 February, the 200MB files have proved popular with many users of file-sharing and chat networks.
However, Microsoft is now taking steps to limit the spread of the code by sending warnings to people who are sharing it.
The software giant said it had written to "some" of the people it had identified as sharing the source code.
File-sharing systems, such as Kazaa, make it relatively easy to trace who is hosting which files by linking them to specific net addresses.
Anyone searching for the source code on file-sharing systems will see a pop-up message alerting them to the fact that downloading the code is illegal.
A spokesman for Microsoft said the letters and messages were simply pointing out that the source code was a trade secret and should not be shared.
"We're asking users to, number one, simply stop sharing and distributing it, and, number 2, to delete and destroy it," said the spokesman.
He added that the letters had already produced "positive responses".
Microsoft is working with the FBI to find out how the code got loose and who was responsible.
Analysis of files found with the chunks of source code suggest that a software firm called Mainsoft could be the source of the leak.
So far the leak is causing Microsoft more embarrassment than anything else because the code in question is quite old.