Malicious Trojan horse software claiming to provide information on topics such as the deadly storms that battered Europe last week infected thousands of computers, according to security vendor Sophos.

The email tries to entice victims into clicking on the malicious attachments by offering reports on topics such as "230 Dead as storm batters Europe", said Graham Cluley, a senior technology consultant with Sophos.

The attachments have names such as Full Story.exe or Full Video.exe. Once they are launched, these files install software that then waits to receive further instructions over the internet, Cluley said. Unlike a worm, this Trojan software does not immediately seek out other targets to infect once it has been installed.

"You think you're reading a news report or you're watching a movie," he said. "It's the age old technique that we've seen since the mid 1990s: here's something you want to look at. Look here!"

Cluley estimates that the Trojan accounted for about one of every 200 emails being sent on the internet last Friday.

Symantec said this particular Trojan variant has been in circulation since last Wednesday, and has counted "thousands" of infections since then. "It's not a small threat, but it isn't an epidemic either," said Dave Cole, a director with Symantec Security Response.

This latest Trojan - rated as a "medium" threat by Symantec - is no more widespread than other similar outbreaks over the past year, but it has raised eyebrows by coming on so quickly.

Some customers were finding that the Trojan was making up as much as 10 percent of all their incoming email on Thursday, said Adam O'Donnell, a senior research scientist at Cloudmark. "It's not as bad as some other ones," he said. "It just sped up so quickly, it caught some people by surprise."