The beta version of free antivirus software from Microsoft has been released in the U.S., Israel, China and Brazil, but not Canada.

As of Tuesday, Microsoft Security Essentials was available for download on the Microsoft website to PC users running Windows XP, Windows Vista or Windows 7 in the selected countries.

Microsoft bills the software as providing "high-quality protection" against both viruses, Trojans, worms, spyware and other malicious software. It updates and upgrades automatically.

The software will be available in Canada when the final version is released in 19 countries during the second half of 2009, Microsoft confirmed Wednesday.

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 in order to harness a larger network of computers. Such "botnets" are then used for activities such as sending spam and stealing personal information.

Microsoft announced in November that it would be releasing security software "tailored to the demands of emerging markets, smaller PC form factors and rapid increases in the incidence of malware." At the time, it said it expected the software, codenamed "Morro," would be available in the second half of 2009.

Microsoft had previously offered security software called Windows Live OneCare by subscription, but planned to discontinue that service on June 30, 2009.