NEW YORK (Reuters) - Microsoft Corp. on Tuesday said it would buy anti-virus software maker Sybari Software Inc. in a direct challenge to Symantec and McAfee Inc., which currently dominate that market.

If Microsoft (Research), the world's largest software maker, bundles anti-virus features into its Windows operating system, security software makers could feel the pressure, analysts said.

"Microsoft's acquisition of Sybari will get them into the (business) anti-virus market specializing in e-mail protection, a negative for Symantec and McAfee," said Sterling Auty, an analyst with J.P. Morgan.

McAfee (Research) shares were down 5.4 percent. Symantec (Research), which recently bought storage software company Veritas to diversify and reduce its dependence on security, was off 3.9 percent.

Auty estimates the Sybari deal will put Microsoft in competition for business representing up to 5 percent of Symantec's revenue and 8 percent of McAfee's revenue.

Closely held Sybari provides anti-virus protection for e-mail systems, primarily Microsoft Exchange and Lotus Notes. Terms of the proposed acquisition were not disclosed.

Sybari filed last year to go public. In its latest filing, in January, it said it might sell 3.35 million shares at $14 to $16 each in an initial public offering. It said it had revenue of $34.1 million and a loss of $2.6 million in the first nine months of 2004.

Shares of Microsoft were up 10 cents at $26.26 near midday on the Nasdaq stock market.

McAfee was down $1.40 at $24.56 on the New York Stock Exchange, and Symantec was off 93 cents at $22.67 on Nasdaq.

Personally, I dont think they should get into the antivirus area, leave that to the companys that are soul based on that. Looks like another money scheme