The change will provide companies with problem resolution, critical and important security hotfixes, access to a security database and the ability to request fixes for new bugs.
The CSA program, which will cover XP SP1, NT 4 and Exchange 5.5, will give customers more options as they facilitate migrations to new versions of Microsoft products.
Not all products will be supported in the program.
Recognizing that companies will continue to use some older software, Microsoft also changed how it charges for its Extended Support program.
Rather than assessing a company-wide price for support of Office or another Microsoft application, companies will only pay on a per-device basis, according to a Microsoft statement.
CSA for legacy products kicks in after the Extended Support program ends, said Microsoft spokesperson Greg Caldwell in an e-mail to internetnews.com.
The new policy follows a six-month period of customer feedback and focus groups on the issue of legacy support.
Product upgrades and lifecycle deadlines are always dicey propositions for Microsoft to negotiate.
Despite Microsoft's wish that people upgrade to the latest version of Windows, 20 percent of companies still use Windows NT 4 for some of their applications, according to Joe Wilcox of JupiterKagan.
As reported last month, a 2005 survey found 61 percent of Americans had upgraded to Windows XP SP2, while only 36 percent of Europeans had updated their systems.
The new policy is only "baby steps," according to Gartner's Michael Silver, who said the software giant needs to specify which legacy applications are included and how long the support can extend.
Silver also said the changes in the CSA program may be preparation for the upcoming end of support for Windows XP SP1, expected Oct. 10.