Microsoft Corp. said it will offer free software to protect PCs from viruses, in a move that could pose new challenges for independent makers of security software.

The Redmond, Wash., company said it plans to introduce its security software, code-named Morro, during the second half of next year in an effort to persuade more users to secure their PCs against spyware, viruses and other forms of "malware." Use of antimalware software is far from universal, especially in emerging markets.

There...