Microsoft is set to acquire Sybari Software, a company which provides anti-spam and anti-virus software to corporates. Sybari currently has about 10,000 clients.
After closing the deal Microsoft will make Sybari's products available under the Microsoft brand, but prices remain unknown.
Stocks of several anti-virus firms, including Symantec and McAfee, dropped after the announcement that Microsoft is going to release commercial anti-virus products.
Symantec shares fell $1.51, or 6.4 percent, to close at $22.09 in Tuesday trading on the Nasdaq Stock Market, while shares in McAfee dropped $2.14, or 8.2 percent, to close at $23.82 on the New York Stock Exchange.
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 $186 million in papers filed to with the Securities and Exchange Commission earlier this month.