Half a million infected computers since the beginning of May has prompted McAfee to declare the latest Trojan threat the most significant malware threat in three years.

A malicious MPEG or MP3 file which has been propagated through popular file-sharing sites has inflicted major damage.

"This is one of the most prevalent pieces of malware in the last three years," said Craig Schmugar, threat researcher at McAfee Avert Labs. "We have never before had a threat this significant that arrives as a media file."

Medium panic

The Trojan has been denoted as a ‘medium’ threat that doesn’t sound like cause for concern but it is the first malware to be given this high a threat level since 2005.

Cyber-criminals have used different file sizes, different languages and different formats to fool people into running the software, and any attempt to play the file will result in ads being served to the infected computer.

McAfee Labs say that the following filenames have been used: "preview-t-3545425-adult.mpg" ; "preview-t-3545425-changing times earth wind .mp3" ; "preview-t-3545425-girls aloud st trinnians.mp3" ; "preview-t-3545425-jij bent zo jeroen van den.mp3" ; "t-3545425-lion king portugues.mpg" and "t-3545425-los padres de ella.mpg"