A Trojan that disguises its malicious content by playing a humorous animation is spreading across the internet, according to security experts.

The Agent-FWO trojan plays the popular "Yes & No" Shockwave video created by Italian animator Bruno Bozzetto, but only after embedding itself on users' computers and downloading further malicious code from the internet.

Hundreds of thousands of people are believed to have watched the film, which was first published online in 2001.

The malware plays the animation as a smokescreen while it silently infects Windows computers, experts at Sophos said.

"The trojan that is playing the animation in this instance is dangerous," said Graham Cluley, senior technology consultant at Sophos. "It is exploiting society's predilection for forwarding humorous animations on to friends and family in its attempt to infect as many people as possible."