Rogue MP3 Trojan streaks across networks.

Since Friday more than half a million computers have been infected, according to McAfee. The Downloader-UA.h Trojan serves ads onto contaminated PCs as part of an apparent money-making scam.

McAfee reckons miscreants loaded hundreds of rigged MP3 and MPEG files onto popular file-swapping services such as Limewire and eDonkey. The files are all named differently (in multiple languages) and vary in size in order to make them appear like legitimate music or video files.

Attempting to play one of the malicious files will trigger the download of an application named "PLAY_MP3.exe" that serves ads onto infected Windows PCs.