A mass mailing worm distributed under the guise of a Christmas greeting is targeting British inboxes. Zaffi D [and variant spellings thereof] first surfaced in April and has already inflicted damage to PCs elsewhere, particularly in South America and other European countries.

Email users should be extra careful about opening messages with the subject headings Happy Hollydays or Merry Christmas. Far from being the Yuletide greeting you expect, clicking on an email generated by Zaffi D results in a grotty image of two manically smiling faces and the installation of software designed to disrupt or damage your PC.

Mark Herbert, CEO of email security company Inty, says the amount of infected mail is subsiding but is likely to rumble on over the Christmas period because of the seasonal message in which it hides itself.

As always, PC Advisor warns you not to open emails sent by people you don’t know. However, Zafi can replicate details from Outlook address books so you should also be on the alert for emails headed Merry Christmas and accompanied by attachments.

Zaffi has also managed to plant itself on PCs via peer-to-peer downloads, ICQ and other instant messaging applications and unsecured wireless networks. Since Zaffi is able to spoof host names and purport to be from someone you know, the malicious mail looks "as innocent as it comes", warns Herbert.

The first instances of attacks in the UK were reported earlier in the week. Security company Inty says its MailDefender software blocked in excess of 20,000 infected messages between midday on Tuesday and yesterday afternoon.