The Trojan horse, which is also designed to steal online banking passwords from Windows computers, was discovered by security experts at Sophos Plc and Symantec Corp.
The virus, called Troj/BankAsh-A and PWSteal.Bankash.A, disables Microsoft AntiSpyware, which is available only as a beta download from the company's website. The malicious code attempts to suppress warning messages displayed by the product and to delete all files within the program's folder.
Experts believe this virus is only the beginning of what will be a salvo of malware attacks on Microsoft security products.
"It's likely that these attacks will continue," Gregg Mastoras, senior security analyst for Sophos, said Thursday. "This is the beginning of a wave of attempts to undermine the effectiveness of this new product."
Beyond targeting Microsoft, the latest trojan also targets users of online banks in the United Kingdom, such as Barclays, Cahoot, Halifax, HSBC, Lloyds TSB, Nationwide, NatWest and Smile. Stealing online banking passwords has become a priority target for criminals.
The British banking industry has responded to the threat by posting on the web information on how online banking customers can secure their computers.
Microsoft AntiSpyware 1.0 Beta, released in January, is the company's first version of the anti-spyware program it acquired through the purchase in December of Giant Company Software of New York.
Earlier this week, Microsoft announced that it was buying Sybari Software, a maker of antivirus, antispam and content-filtering technologies. Sybari, East Northport, N.Y., is a longtime Microsoft ISV partner that makes security add-ons for Exchange Server and other products.
In an odd twist, security companies that are partners with Microsoft in selling software for Windows computers now find themselves becoming competitors with the Redmond, Wash., software maker. In addition, in related news, anti-virus vendors' stock prices have taken a beating since Tuesday, when Microsoft announced the purchase of Sybari.
In security, however, Microsoft is seen by competitors as the new kid on the block.
"We believe that we have a certain expertise that we've built up for a number of years," Mastoras said. "It's going to be difficult for anyone to come in new and duplicate the level of security that we'll be able to offer."