The deal for Sybari Software, along with word that Microsoft is gearing up to release its first set of commercial antivirus products, could hurt security companies including Symantec and McAfee, whose stock prices fell.

Terms of the deal, the latest in a series of security-related purchases by Microsoft, were not disclosed. Sybari is privately held but had been planning an initial public stock offering. The company estimated its market value at up to US$186 million in papers filed with the Securities and Exchange Commission earlier this month.

The acquisition will produce Microsoft's first official separate paid antivirus offering, said Mr Mike Nash, corporate vice-president of Microsoft's Security Business and Technology Unit.

The company has about 10,000 clients. Its software scans businesses' e-mail messages to try to ward off attacks.

Mr Nash said Microsoft would make the Sybari-based product, geared towards business customers, available under the Microsoft brand soon after the deal closes.

Microsoft has not yet said how much the new product will cost.

In an interview, he said Microsoft would subsequently release other products, for both consumers and business users, aimed at protecting computer desktops from Internet-based attacks. He could not yet say exactly when those would be released, however.

Sybari is just the latest company Microsoft has bought so it can make its own security products.

It purchased a Romanian antivirus firm, GeCAD Software, for an undisclosed amount in 2003. Then, in December, it bought Giant Company Software, which makes tools to remove spyware, software that monitors a person's computer habits, slows down computers, triggers pop-up ads and worse.

Earlier this year, Microsoft began offering free programs to remove viruses and spyware. It plans to eventually charge for more sophisticated antivirus tools, and it has said it may one day charge for spyware removal products as well.