Microsoft Corp is getting ready to unveil a long-anticipated free anti-virus service for personal computers that will compete with products sold by Symantec Corp and McAfee Inc.

A Microsoft spokesman said on Wednesday that the world's biggest software maker is testing an early version of the product with its own employees. Microsoft would "soon" make a trial version, or product beta, available via its website, he added, but declined to provide a specific date.

Symantec shares fell 0.5 per cent on Nasdaq and McAfee fell 1.3 per cent on the New York Stock Exchange, while Microsoft was up 2.1 per cent. The Nasdaq composite index was down 0.47 per cent.

Investors are closely monitoring the free service, code-named Morro after Brazil's Morro de Sao Paolo beach, amid concern it could hurt sales of products from Symantec and McAfee, which generate billions of dollars of revenue a year protecting Windows PCs from attacks by hackers.

"It's a long-term competitive threat," said Daniel Ives, an analyst with FBR Capital Markets, though he added that the near-term impact was minimal.

Microsoft has said that Morro will offer basic features for fighting a wide range of viruses, which would likely make it comparable to low-end consumer products from Symantec and McAfee that cost about $40 per year.

Their top-selling products are security suites that come with features including encryption, firewalls, password protection, parental controls and data backup.

Three years ago, Microsoft entered that market with Live OneCare, which turned out to be a commercial flop. It announced plans in November to kill that product suite, saying it would launch the free Morro service by the end of 2009.

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