It is not often that a virus is discovered and in the very same day it is listed as one of the top five security threats by experts. This virus is one of those, and can affect anyone using recent versions of Microsoft mail products.

The worm, which is being dubbed "mimail," attempts to exploit a vulnerability in Internet Explorer that allows a script to be executed by an infected computer. The worm then tries to use that script to mass e-mail itself, potentially clogging mail servers or slowing down networks, according to antivirus company Symantec.

The e-mail that carries the worm has "your account" in the subject line, according to Symantec, and the body reads, "Hello there, I would like to inform you about important information regarding your e-mail address. This e-mail address will be expiring. Please read attachment for details."

It is then signed "Best regards, Administrator" and contains an attachment labelled "message.zip" that carries the malicious code.

In terms of its method, the mimail bug is somewhat similar to other mass-mailing worms, said Sharon Ruckman, a senior director at Symantec Security Response. What's trickier than usual, she said, is the way the e-mail that carries the worm tries to get people to open the attachment.

"The social engineering aspect (is) a lot more serious," Ruckman said. "You believe it was the administrator from your own domain, whether that is your company or your ISP."

Also of note, Ruckman said, is that the mass e-mailing code is contained in an HTML file, a type of file not normally associated with executing programs. Ruckman recommended that corporations either delete the attachments at the server level or block messages with the "your account" subject line.

You can protect yourself from this virus by ensuring that you regularly patch your system - www.windowsupdate.com is the best way - and if you think you may be infected, download the free removal tool at the Symantec Antivirus Research Centre.