The leak, the second major embarrassment for Microsoft in a week, followed warnings that new vulnerabilities in the Windows operating system could be exploited by hackers.
Microsoft admitted last week that about 15% of the source code of Microsoft NT and Windows 2000 had been released on to the internet.
The code, which is closely guarded by Microsoft, could be exploited by hackers to attack Windows servers, security experts warned.
"We do not know what code has been released. However, the fact that Microsoft held a press conference suggests that it is something more than a minor annoyance," said security consultant Richard Starnes.
"If it is core operating code, communication services code, or any code dealing with Microsoft security, there should be a level of concern," he said.