Anti-virus vendors' stock prices have taken a beating since Tuesday, when Microsoft announced its third security-related acquisition, the purchase of Sybari Software.

Sybari, a long-time Microsoft partner, makes anti-virus, anti-spam, and content-filtering add-ons for Exchange Server, Lotus Notes, and other products. Third-party security firms such as Symantec and McAfee have been aggressively moving on the server market as they expand from earlier consumer-centric sales into the enterprise.

"Microsoft's entry into this market will challenge anti-virus vendors such as Symantec, McAfee, and Trend Micro, which derive substantial revenue from upselling enterprises to anti-virus product suites that include desktops and servers," said a trio of Gartner analysts Wednesday in a briefing posted to the research firm's Web site.

Judging by Wall Street's reaction, investors agree.

Since the close of the market Monday, Symantec's stock has fallen 10.6 percent, to $21.10 (as of mid-morning, PST, Thursday). McAfee's price as dropped even more: 11.3 percent, to $22.92. Another anti-virus vendor, Trend Micro, has seen its shares slip 7.6 percent, to $43.11.

"The Sybari architecture will also enable Microsoft to plug in its own anti-virus engine and allow other providers' engines to be used, as well," the Gartner analysts continued. "Gartner believes Microsoft's AV engine, along with its signature service, will be the foundation of Microsoft's forthcoming desktop offering."

The analysts -- Neil MacDonald, Arabella Hallawell, and Maurene Caplan Grey -- said that there's an 80 percent chance Microsoft will add Sybari functionality to Exchange 2003 in Service Pack 2 (SP2) by the third quarter.

Earlier in the week, Symantec was relatively defiant that its market was not threatened by the Microsoft acquisition. In a statement it released Tuesday via e-mail, a Symantec spokeswoman said, "[Sybari] only provides the plumbing that enables engine integrations with Microsoft Exchange and other gateway products, but not the core security service and response technologies themselves.

"This acquisition does not provide Microsoft with the security and anti-virus response infrastructure necessary to support the virus protection needs of enterprise customers," she added.

Sybari is the third security-related acquisition that Microsoft's made since mid-2003. In June of that year, the Redmond, Wash.-based developer acquired anti-virus assets and technologies from Romania-based GeCAD Software. Last December, Microsoft purchased Giant Company Software, an anti-spyware vendor, and almost immediately rewrapped Giant's software to put out the beta of Windows Anti-Spyware.