Microsoft Corp. (MSFT) on Tuesday will release a preliminary version of a free antivirus tool, shifting its strategy for computer security protection, which underscores the growing popularity of free security tools.

Microsoft Security Essentials, which is designed to identify and destroy viruses and other malicious software, will replace Microsoft's OneCare, a $50-per-year subscription security service.

The move into free antivirus protection from the world's largest software maker could pose a challenge to the large security software makers, such as Symantec Corp. (SYMC) and McAfee Inc (MFE).

Theresa Burch, a director of product marketing for Microsoft Security Essentials, said the product is primarily designed for emerging market customers who in many cases may not have credit cards or other means of paying for tools via the Internet. The tools are stored locally on the user's computer, but users don't have to re-subscribe to get ongoing protection, Burch said.

Research shows that the proportion of people downloading free antivirus software increased in 2009 over 2008.

"There's an increasingly technically savvy population who are happy to cobble together their own security packages," said Jonathan Penn, a security software analyst for Forrester Research. "There's also a recognition that whether they use free or paid software, consumers won't ever be completely safe."

Both Symantec and McAfee argue that they have more comprehensive antivirus protection in their consumer antivirus tools, such as Symantec's Norton 360 and McAfee's Total Protection. Mike Plante, a product marketing specialist at Symantec, said in reviews, Microsoft's product under-performs not only Symantec's products but the existing free products.

While few analysts think consumers will abandon the use of paid antivirus tools in droves, Symantec and McAfee "have the most to lose," from free antivirus tools, Forrester's Penn said.

Symantec gets around one third of its $6.2 billion in annual revenue from consumer antivirus tools, but these tools are among the most profitable of the company's products, analysts say. Symantec is the world's largest security software maker, with around a third of a market worth $10 billion in 2007. McAfee, which is expected to generate around $1.8 billion in 2009 sales, gets about 40% of its sales from consumer antivirus products.

-By Jessica Hodgson, Dow Jones Newswires; 415-439-6455; jessica.hodgson@dowjones.com