A new e-mail worm is spreading rapidly on the Internet, clogging e-mail servers and staging an attack on the Web site of Unix vendor The SCO Group, anti-virus software vendors said.The worm surfaced Monday and has been given several names by anti-virus software vendors, including Mydoom, Novarg and Mimail.R. Experts don't all agree on the worm's payload, but they do agree that it is spreading faster than Sobig-F, the worm that topped the charts for the most widespread e-mail worm last year."It has been moving very quickly for the past three hours and has been generating a hell of a lot of e-mail," Vincent Gullotto, vice president of the Anti-Virus Emergency Response Team at Network Associates, said Monday afternoon. Some businesses have shut down their e-mail gateways to block the worm, he said.This worm has taken off like a rocket, with well over 20,000 interceptions within just two hours of it being discovered, Ken Dunham, director of malicious code at Internet security company iDefense, said in a statement via e-mail.Massive spreading of the worm slowed down performance of the top 40 U.S. business Web sites Monday afternoon, according to Keynote Systems, a San Mateo, Calif., Web performance monitoring firm. The average time for a site to load exceeded four seconds, while they normally load in two to three seconds, Keynote said in a statement.The worm arrives as an e-mail with an attachment that can have various names and extensions, including .exe, .scr, .zip or .pif. The e-mail can have a variety of subject lines and body texts, but in many cases it will appear to be an error report stating that the message body can't be displayed and has instead been attached in a file, experts said.The sender's address can be spoofed, meaning that the message could appear to be from a colleague, friend or the e-mail system administrator."This is something you might see from a mail system, so you click on the attachment," said Sharon Ruckman, senior director for Symantec Security Response. Only users of computers running Microsoft's Windows are at risk, according to Symantec.Both Network Associates and Symantec agree that when the attached file is executed, the worm scans the system for e-mail addresses and starts forwarding itself to those addresses. If the victim has a copy of the Kazaa file-sharing application installed, it will also drop several files in the shared files folder in an attempt to spread that way.Symantec also identified more malicious acts. The worm will install a "key logger" that can capture anything that is entered, including passwords and credit card numbers, Ruckman said. Furthermore, the worm will start sending requests for data to www.sco.com, the Web site of The SCO Group, which could result in the Web site going down if enough requests are sent, she said.The denial-of-service attack on the SCO Web site is programmed to occur between Feb. 1 and Feb. 12, according to a Symantec statement on its Web site late Monday evening.Anti-virus vendors Trend Micro and F-Secure report that the worm installs a "backdoor," potentially allowing an attacker access to the infected system.SCO has noticed that its Web site performance has intermittently slowed, but it is too early to say if there is an attack on the site, said SCO spokesman Blake Stowell. "It may be showing the early stages of a\u00a0denial-of-service attack," he said. SCO has enraged the open source community by claiming that the Linux operating system contains software that violates SCO's intellectual property, and has been the subject of various attacks on its Web site.The Mydoom worm so far has spread mostly in the U.S., after it was first detected in Russia, e-mail filtering company MessageLabs reported on its Web site late Monday night. Information from Trend Micro at the same time showed the worm is also spreading in Australia, New Zealand and Japan.Anti-virus software vendors urge users to update their anti-virus software and be careful when opening e-mail attachments. "If you're not expecting an e-mail, don't open it," Symantec's Ruckman said.Network Associates' Gullotto expects the worm to keep causing headaches for a while. "It will be a couple of days before we're going to get to the point that it won't have any impact. It has a full head of steam, there are hundreds of thousands of e-mails and we may see well into the millions (of e-mails), and possibly hundreds of thousands of machines infected," he said.