An email worm that masquerades as a new year greeting to catch out workers returning to work is spreading quickly.

Messages containing files named 'postcard.exe' and 'postcard.zip' actually hide a mass-mailing worm called Dref-V, antivirus companies have warned. 

"This started spreading on 30 December and accounted for a whopping 93.7 per cent of all infected email in the last two days of the year," Graham Cluley, senior technology consultant at Sophos, told vnunet.com. 

"The danger is that people returning to work today may be opening their email and launching attachments without taking proper care over their security."

The attachment affects Windows computers by downloading malicious software from the internet and turning off security software such as antivirus products.

Dref-V then looks for open mail proxies to send further spam emails and infect other computers.

However, Cluley said that the good news is that most antivirus products have been updated, although he warned that there might be a few companies which have been too laid back and are at risk today.