The email is entitled "Here you have" and holds within a link that appears to direct users to a PDF document.
In fact the link contains no documents but, if clicked, will enable the virus to access the user's Outlook address book and email itself to every contact contained within.
sending pornographic emailsIt will also attempt to disable any security programmes by deleting them, allowing it to remain hidden on the computer's hard drive.
The worm can also be passed on by unsecured links between computers sharing a network.
BBC News reported that companies including Nasa, AIG, Disney, Procter & Gamble and Wells Fargo are all having difficulties preventing the worm from spreading through their systems, with employees receiving hundreds of copies of the email.
The website on which the worm was based was shut down on Thursday evening, but it is expected that other forms of the virus will continue to spread.
Kaspersky, the security company, said the worm targeted Outlook in the same way as previous viruses such as the ILoveYou bug, which spread across the world in 2000.