A new Internet virus (search) was spreading fast throughout Asia, Australia and Europe but computer security experts were divided on the seriousness of the threat from the "Bagle" worm, and reckoned home users were most at risk.

Experts expected some impact in the United States when people returned to work Tuesday after a holiday weekend.

The "Bagle" or "Beagle" worm arrives in an e-mail with the subject "hi" and the word "test" in the message body. If the accompanying attachment is executed, the worm is unleashed and tries to send itself to all e-mails listed in the user's address book.

Sometimes the attachment is designed to look like a Microsoft calculator, said David Perry, spokesman for antivirus software firm Trend Micro Inc. (search).

The virus only affects machines running Microsoft Windows operating systems.

"It's clumsy," Perry said from Lake Forest, Calif., adding that most people knew better than to click on an attached calculator: "I don't get e-mails with calculators in it, do you?"

The worm started spreading on Monday and most corporations have already protected themselves against it, Perry said.

Carey Nachenberg, chief architect of Symantec Research Labs (search) in Cupertino, Calif., said home users, not corporations, were most at risk because companies had protected themselves quickly.

"We could see this fizzle out in several days," Nachenberg said. "Or we could also see a lot of people infected" if they don't update their antivirus software.