Microsoft has agreed to provide code so that software companies can provide their own security add-ons to its Vista operating system.
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.
"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.