On Wednesday, antivirus company Sophoshorse, dubbed "Bankash-A," which suppresses warning messages displayed by and deletes all of the files in the program's folder. The Trojan also steals passwords and online-banking details from Windows users.
"Microsoft is actively investigating new public reports of a criminal attack, known as the 'Bankash-A Trojan'," the company wrote in an e-mail statement. "Microsoft is not aware of any significant customer impact resulting from the Trojan. Microsoft continues to recommend customers evaluate the Microsoft AntiSpyware beta and encourage customers to follow the three steps to help keep your PC protected (at) www.microsoft.com/protect."
The software giant is also offering free support to victims of the Trojan.
"Customers who believe they may have been affected should contact Product Support Services," the company wrote. "Product Support Services in North America can be contacted for help with security update issues or viruses at no charge by using the PC Safety line (1-866-PCSAFETY). International customers can receive the same level of support by using any method found at this location: http://support.microsoft.com/."
Microsoft added that people should consult their local law enforcement agencies if they have been infected by the Trojan.
"Customers who believe they have been attacked should contact their local FBI office or post their complaint on www.ifccfbi.gov. Customers outside the U.S. should contact the national law enforcement agency in their country."
The Trojan targets users of U.K. online banks such as Barclays, Cahoot, Halifax, HSBC, Lloyds TSB, Nationwide, NatWest and Smile.
On Wednesday, Sophos said the Trojan was the first piece of malicious ware to target the anti-spyware product, which is still only available in a test version.
"As Microsoft's product creeps out of beta and is adopted more by the home user market, we can expect to see more attempts by Trojan horses, viruses and worms to undermine its effectiveness," said Graham Cluley, senior technology consultant for Sophos.
Dan Ilett of