Microsoft's free antivirus program, Security Essentials, became available for download from the company's website Tuesday.
Microsoft bills the software as providing "high-quality protection" against viruses, Trojan horses, worms, spyware and other malicious software. It updates and upgrades automatically.
Microsoft said its free program is not meant to cut into the business of commercial antivirus programs, by companies such as McAfee and Symantec, but will appeal to people who don't already have security software on their computers.
The beta version of the program became available in the U.S., Israel, China and Brazil in June.
The program is available for Microsoft Windows XP and Windows Vista operating systems, as well as Windows 7, which goes on sale Oct. 22.
Microsoft's Windows operating systems have long been a main target of malicious code writers. Security experts have said that is largely because they have a much greater market share than Mac OS or Linux.
Malicious code is often used to turn computers into "zombies" controlled by criminals, who typically want to infect as many computers as possible to harness a larger network of computers. Such "botnets" are then used for activities such as sending spam and stealing personal information.