Microsoft has agreed to provide code so that software companies can provide their own security add-ons to its Vista operating system.
Microsoft's new operating system is keenly awaited
Rivals had previously complained they were locked out from the security system for Vista.
Experts warned the lack of access increased the risk of malicious hacks and viruses.
The u-turn has been welcomed although the timeline for the provision of data is still being hammered out.
Microsoft chief executive Steve Ballmer said it would deliver it "today, Seattle time, not Rome time".
The news that the software giant will now allow companies such as McAfee and Symantec access to the kernel of the 64-bit version of Vista has been met with cautious approval.
"While we are encouraged by their statements and are hopeful their actions will indeed lead to customers being allowed to use whatever security solutions they would like on the Vista operating system, the operative question is exactly when will the final detailed information be made available to security providers?" read a statement from Symantec.
It is concerned that, with Vista due to ship to businesses within the next few weeks, time is running out.
Earlier this month, security firm McAfee took out a full-page advert in the Financial Times to alert readers to its worries about the way Microsoft was handling the release of its new operating system.
"Microsoft seems to envision a world in which one giant company not only controls the systems that drive most computers around the world but also the security that protects those computers from viruses and other online threats," the advert said.
It is holding judgement on Microsoft's change of heart until it has seen the details, a spokesman for the firm said.
Not all security firms have had issues with Microsoft. Security experts Sophos will release its Vista-compatible product next month.
"From our point of view, communication has gone well and we have not been obstructed in any way," said Graham Cluley, senior consultant at Sophos.
"Microsoft is getting better at securing its operating systems although this is not the end of the road. Vista is not going to be the answer to the virus problem," he said.
All eyes are on Microsoft in the run-up to the January release of Vista, including the European Union which is checking there are no issues of monopoly abuse with the new operating system.
EU Competition Commissioner Neelie Kroes has already had major run-ins with Microsoft over Vista. She has called for changes to the European edition of the operating system to better open it up to rival software providers.
Microsoft has enjoyed a somewhat rocky relationship with the EU since it was fined 497m euros ($629m; £335m) for anti-competitive behaviour in 2004, and was further hit with a 280m-euro penalty earlier this year for failing to meet demands that it share technical information with rivals.