The volume licence key allows installation of Windows Server 2003 on multiple systems without the activation process required for single licences. Volume licence keys are meant for corporate users.
A Microsoft spokeswoman confirmed that a volume licence key had been leaked to the internet and said the company was investigating the situation.
Microsoft is trying to determine whether the key was leaked from within Microsoft or by a customer of the company.
"We're still in the investigative stages," the spokeswoman said.
Along with the key, several copies of Windows Server 2003 have been posted illegally on the web for download. The software will be released officially on 24 April.
It is not unusual for a registration key to leak online. There are hundreds of websites that offer registration keys, key generators and software cracks, which is software with copyright protections disabled.
With the release of its Office XP and Windows XP products, Microsoft instituted a product activation feature that was designed to thwart software piracy.
The Microsoft technology requires all users to "activate" their copy of Windows XP soon after they purchase it. This process "locks" a product identification number assigned to each copy of Windows XP to the PC it is installed on.
Information from the machine on which XP is installed are collected and used to generate a unique activation code based on the machine's configuration.
Because no activation is required for customers with a volume licence key, however, Microsoft cannot disable the key remotely, the spokeswoman said. She could not comment on whether there is a ceiling to the number of Windows Server 2003 copies that can be created with a volume licence key.
However, Microsoft can stop customers using the key from receiving future updates and service packs and exclude systems using the key from software updates, which are essential to keep the system secure.