Under the global initiative, licensed companies will be able to view specific areas of Microsoft's source code - the blueprint for all software - for Windows 2000, Windows CE and .net technologies.
One aim is to help to speed up the process by which users iron out glitches in applications running on Windows operating systems by helping them to understand the source code.
The opening up of Microsoft's source code could also help to improve the performance of a wide range of hardware and handheld devices that run on Microsoft operating systems.
Phil Cross, Microsoft UK's developer marketing manager, said, "[The new source code access] allows developers to see the code but does not allow them to change and distribute it.
"Our aim is to make as much of the source code available as necessary to help the developers make their applications work better with Windows."
By allowing some application developers and original equipment manufacturers to scrutinise the source code, Microsoft hopes the hardware will be tied more tightly to the operating system.
In the case of Windows CE such access is essential because the microprocessor varies more widely than in the Intel-dominated desktop and notebook market. For PocketPCs, there is an obvious benefit in harmonising the operating system and the chip to ensure that the maximum performance can be achieved from the limited resources available.
The latest source code announcement builds on an ongoing trial in the US for enterprise customers as well as a limited pilot scheme in Europe.
However Microsoft is not joining the Linux-driven open source movement where all source code is freely available to anyone.
To qualify for the privilege of seeing Microsoft code, a company must make a case for why it needs to see the code and Microsoft will judge each case on its merits.
Sanctioned companies will then be issued with a non-disclosure agreement and, on signing up, will be sent the relevant code.
Some parts of Windows will not be made available because they are based on licences from other companies which supply code for Windows or because they are subject to US government constraints on revealing details of encryption systems.
Analysts were unimpressed by the Microsoft announcement and argued that only a limited number of companies would have the necessary IT skills and resources to benefit from the offer.
"Most people who take up the offer will get little benefit because of the high expertise required," said Dan Kusnetzky, IDC's vice-president for systems software research.
Microsoft UK is setting up the channels to handle requests for code access.
Eric Doyle and Daniel Thomas