San Jose, Calif. ? A malicious program attached to seemingly innocuous e-mails was spreading quickly over the Internet on Monday, clogging network traffic and potentially leaving hackers an open door to infected personal computers.

The worm, called “Mydoom” or “Novarg” by antivirus companies, appears to be an e-mail error message. A small file is attached that, when launched on computers running Microsoft Corp.’s Windows operating systems, can send out 100 infected e-mail messages in 30 seconds to e-mail addresses stored in the computer’s address book and other documents.

The attack was first noticed Monday afternoon. Within hours, thousands of e-mails were clogging networks, said Vincent Gullotto, vice president of Network Associates’ antivirus emergency response team.

Besides sending out e-mail, the program appears to open up a backdoor so that hackers can take over the computer later.

“As far as I can tell right now, it’s pretty much everywhere on the planet,” Gullotto said.

Symantec, another antivirus company, also said the worm appeared to contain a program that logs keystrokes on infected machines. It could collect username and passwords of unsuspecting users and distribute them to strangers.

Network Associates did not find the keylogging program.

The worm also appears to deposit its payload into folders open to users of the Kazaa file-sharing network. Remote users who download those files and run them could be infected.

Symantec also found code that appeared to target The SCO Group Inc., which claims some of its intellectual property has ended up in the Linux operating system and is threatening lawsuits. SCO’s Web site, which has been targeted in the past, was available but sluggish Monday.

The computer security firm Central Command confirmed 3,800 infections within 45 minutes of initial discovery.

“This has all the characteristics of being the next big one,” said Steven Sundermeier, Central Command’s vice president of products and services.

Unlike other mass-mailing worms, Mydoom does not attempt to trick victims by promising nude pictures of celebrities or mimicking personal notes. Instead, one of its messages reads: “The message contains Unicode characters and has been sent as a binary attachment.”

“Because that sounds like a technical thing, people may be more apt to think it’s legitimate and click on it,” said Steve Trilling, Symantec’s senior director of research.

Subject lines also vary. The attachments have “.exe,” “.scr,” “.cmd” or “.pif” extensions, and may be compressed as a Zip file.

Microsoft offers a patch of its Outlook e-mail software to warn users before they open such attachments or prevent them from opening them altogether. Antivirus software also stops infection.

Christopher Budd, a security program manager with Microsoft, said the software giant was working with other companies to learn more about the worm, but that, as of yet, the information about the worm was still “very spotty.”

The Redmond, Wash.-based company was encouraging users to take precautions such as using an Internet firewall and using up-to-date antivirus software.