The Goker worm is not destructive and has not yet infected many systems, but as with any mass-mailer worm, could become a nuisance as unsuspecting users spread it.

Like other mass-mailing worms such as the Anna Kournikova worm or Badtrans, Gokar spreads through Microsofts's Outlook and Outlook Express email clients.

It activates when a user clicks on an attachment sent with the infected message, according to antivirus firms Symantec, F-Secure, and Trend Micro.

Infected email arrives in user inboxes with dozens of combinations of different subject lines, body messages, and filenames, though each attachment will end with the PIF, SCR, EXE, COM or BAT extensions, according to the companies.

When the attachment is double-clicked, the worm installs a file called Karen.exe on the infected system and mails itself to all addresses listed in the computer's address book.

The worm then runs every time the infected computer is booted up. Whether a system is infected or not can be determined by searching for the Karen.exe file.

The worm also uses the chat program Internet Relay Chat (mIRC), according to the companies.

Gokar searches the infected PC for the mIRC application, and if it finds it, attempts to infect IRC users in the same discussion, or channel, as the infected system whenever the application is started, according to Trend Micro.

Lastly, if an infected system is running Microsoft's Internet Information Services Web server software, the worm will modify the default eeb page on the system and offer users visiting the site a chance to download the worm, according to F-Secure.

An infected website will be changed to display the text "We are forever" and point users to a link to download a file called web.exe, which contains the Gokar worm, according to Symantec.

The Nimda worm also defaced websites and downloaded files to the computers of users viewing the site.

Unlike Nimda, which automatically downloaded a file through the browser, Gokar requires that the user click a link to download the worm.

Users should check with their antivirus companies for software updates.

Companies are urged to block attachments, especially exe, scr, and pif files, at their mail gateways to avoid infection.