Security firms are warning about the W32/Nyxem-D worm which pretends to be a pornographic email attachment.

Emails containing the worm come with a variety of subject lines, including 'School girl fantasies gone bad', 'Fwd: Crazy illegal Sex!' and 'Arab sex DSC-00465.jpg'. The attachment is a .PIF file with a similar variety of salacious file names.

"Companies should educate their users to practise safe computing, including never opening unsolicited email attachments and discouraging the sending and receiving of joke files, pornography and funny photographs and screensavers," said Graham Cluley, senior technology consultant at Sophos.

"This worm feeds on people's willingness to receive salacious content on their desktop computer, but they could be putting their entire company's data at risk."

Once activated, the worm seeks to shut down all firewall and antivirus software and to harvest email addresses from the host computer. It then emails itself out using its own SMTP engine, and will seek to update itself periodically from the web.

At the time of going to press this virus is the most prevalent being reported, according to Sophos, accounting for a quarter of all global infections.