The latest variant of the Sober worm is aiming for the top virus of the year spot, with a staggering one in 14 e-mails circulated on the Internet containing this morning, according to Sophos.

Around 85 percent of all viruses reported to Sophos are sober.z, up from around 60 percent last week, said Graham Cluley, senior technology consultant. Sober.z is the third most prevalent virus for the year so far, behind zafi.d and netsky.p.

"It isn't slowing down," Cluley warned. "At the moment, it's getting worse."

Sober.z first appeared around 22 November using various forms of social engineering to trick users into executing the attachment. Messages purporting to be from the FBI warned recipients that they have been visiting illegal websites and asked them to read a list of attached questions.

Other versions pretended to be from the CIA or offered video clips of Paris Hilton and Nicole Richie. While most anti-virus vendors have updates, the social engineering ploys are still effective.

"I think the problem is there are some people who simply don't have protected computers and are spewing this out to other people," he said. The worm, which is believed to have originated in Germany, scans hard drives for e-mail addresses and also tries to shut off security software.