There is virulent virus on the loose, codenamed SoBig, according to antivirus firm MessageLabs.

The worm, which was first discovered last week, has spread so quickly that by the beginning of this week it had become the third most prevalent virus on the internet, with MessageLabs recording over 44,000 instances of the bug.

SoBig is a mass-mailer worm that affects Windows 95, 98, Me, NT, 2000 and XP. It arrives in inboxes from '[email protected]' with the subject headings: 'Re: Here is that sample'; 'Re: Document'; 'Re: Movies'; and 'Re: Sample'. The email also contains a PIF attachment.

Once the attachment is opened the program searches through all files containing email addresses. It then connects to a website, downloads a text file containing another web address and attempts to download and run another program. Like most email worm viruses it also harvests email addresses and forwards itself on to them.

MessageLabs suspects the worm is a backdoor Trojan which allows the hacker to take control of the user's PC. It advises all users to delete the message as soon as they receive it.

Its main target is Outlook Express users.

More information and patches can be found on MessageLabs website.