The move will be seen as a response to the popularity of the Linux operating system which has gained powerful support from suppliers including IBM, Hewlett-Packard and Compaq over the past few years.
Microsoft, however, is adamant that it is not adopting Linux's open access approach to its software code.
Microsoft's shared source programme was outlined in a speech made last week in the US by Craig Mundie, Microsoft's senior vice-president of advanced strategies.
But after unveiling its details, he moved to distance Microsoft from the open source movement and re-affirmed Microsoft's proprietary commercial software model, arguing that it would provide a better product for users than software based on pure open source code.
"There are elements to open source software that are good for our customers and partners, such as its fostering of the development community, improved feedback loops and augmented debugging," Mundie said.
"But there are also elements to be avoided such as a strong possibility of unhealthy forking, interoperability concerns and significant licensing issues," he added.
Forking is when the code base for software splits in many directions, as the original Unix experienced when different "flavours" ruined interoperability.