According to a warning posted on security company Panda Software's Web site, virus writers have released licence-protected multimedia files containing Trojan horses that overcome the anti-piracy features in version 10 of the Media Player and Windows XP SP2.
Normally, when a user tries to play a licence-protected file but doesn't own the necessary licence, Media Player goes onto the Internet to find a site where they can buy one.
This Trojan, dubbed WmvDown.A, subverts this process by sending the player to a malicious Web site (serve.alcena.com) from which it downloads malware.
According to Panda Software, the Trojan is easy to spot once it has run. After connecting to the Internet, it displays the message: "Thanks for downloading this file. Click Play to listen."
A Microsoft spokesperson was not immediately able to comment on Panda's claims.