The people behind the recent Hackarmy Trojan attacks have sunk to a new low in their bid to hijack people's PCs, claims security firm Sophos.

Hackarmy has been posting messages to newsgroups to entice users to click on files that will infect their PCs with a Trojan programme, thus allowing hackers to hijack the infected PC. Compromised PCs can be used to launch denial of service attacks or to steal personal information.

The latest message, posted to thousands of newsgroups, claims that American civilian Nick Berg, who was kidnapped while working in Iraq and beheaded in May by militants linked to Al Qaeda, is still alive.

The posting reads: "Conspiracy theories of Nick Berg being alive and well in Iraq have today been proven true. [Arabic news network] Aljazeera have released video footage of the supposedly beheaded American captive.

"The clip was first 'discovered' on an Islamic website in Malaysia and has now been released by American Journalists collaborating with Aljazeera. The evidence speaks for itself and viewed firsthand here."

But the file supposedly containing the video clip proving Berg is alive in fact contains the same Trojan as the earlier 'suicide note' from Arnold Schwarzenegger and Osama Bin Laden 'death' photographs.

"This is really sick to take advantage of a real tragedy. His family has suffered enough," said Graham Cluley, senior technology consultant at Sophos.

"But there are a lot of conspiracy theories about Nick Berg on the internet and people are curious and want to read more, so the hackers are feeding on this.

"The trouble is, although we are starting to see warnings on newsgroups going up, it needs someone tech savvy [to recognise it], and all the hackers need are a few people from the thousands of newsgroups to fall for it."