Microsoft's move is an attempt to placate European regulators who are wary of the software giant, having decided in 2004 that the company uses its dominance of the PC operating system market to stifle competitors.
The firm said it will show the likes of Symantec and McAfee the kernel of the 64-bit version of Vista so that they know how to build products for it. They will now have two or three months to develop their products for Vista.
The European Commission says it will keep a beady eye on Vista to see what happens.
Vole-in-chief Steve Ballmer said the code would be made available to the security firms today, adding that he now believes the company is in compliance with the European Commission's demands, according to Reuters.