A new Trojan that hijacks Google text ads and replaces them with ads from a different provider has been picked up by BitDefender.

The antivirus company has identified the threat as Trojan.Qhost.WU which modifies the infected computer's host file, a local storage for domain name/IP address mappings.

The infected machine's browser then reads advertisements from a server at the replacement address rather than from Google.

"This damages users and webmasters because it takes away viewers and thus a possible money source from their websites," said BitDefender virus analyst Attila-Mihaly Balazs.

Users are advised to let antivirus software delete the Trojan.