Microsoft says its new computer security program can be downloaded starting today.
Microsoft Security Essentials, as the free antivirus software is called, has been available in a beta test version since June.
The software updates daily to stay current with the latest malicious programs, which can steal passwords or turn PCs into spam servers. Microsoft says it won't make computers run slower.
Microsoft has said it isn't out to steal business from companies like McAfee and Symantec, which make popular antivirus programs with more features.
Microsoft says it hopes the free program will appeal to people who don't already run antivirus software.