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The outbreak of an Internet infection that disabled thousands of PCs this week appears to be waning, computer security experts said Wednesday.

While the worm continues to harass businesses and computer users around the world, the number of computers affected dropped by 30 to 40 percent on Tuesday from the day before, according to Symantec, a Cupertino maker of antiviral software.

"The virus has hit its natural limit," said Vincent Weafer, senior director of domestic security responsible for Symantec. "It peaked on Monday, and then we started to see a decrease."

The computer pox, called Blaster, MSBlast, LovSan and other nicknames, has spread rapidly in the past few days from one computer to another via the Internet, exploiting a flaw in newer versions of Microsoft Windows.

The experts said computer security experts have successfully beaten back the worm by patching the security hole on individual PCs and setting up blockades on corporate networks.

Still, Weafer predicted the worm will continue to remain a nuisance for weeks or months to come. Experts urged home and small businesses to download the latest security patch at www.microsoft.com/security/incident/blast.asp.

The disease affects users running several recent versions of Windows, including Windows 2000 and Windows XP. But it won't disrupt computers running Windows 95, 98 and ME, prompting at least one expert to joke that it was only half a virus.

To help ward off future attacks, Microsoft said it would begin shipping its Windows XP software with its protective network firewall completely activated, to make PCs less vulnerable to attacks, the New York Times reported.

Meanwhile, customers reported scattered outages at some companies. And several major corporations said they raced to contain the infection this week.

At Levi's corporate headquarters in San Francisco, employees who showed up to work on Tuesday morning were greeted with signs plastered around the building warning them not to log onto their computers.

Computer technicians then spent the day painstakingly updating the software on every computer at the clothing maker. Some employees' computers were back up and running by 9 a.m. or earlier. Others were disconnected all day.

But Levi's spokesman Jeff Beckman said the worm didn't affect any of its cash registers, allowing it to keep its stores up and running as usual. And Beckman said office employees were still able to work the old-fashioned way, giving them a break from the normal deluge of e-mail and electronic paperwork to return phone calls.

Still, a number of major firms, including, Citibank, Clorox, Verizon Wireless, AT&T, Hughes Electronics's DirecTV unit and McKesson, reported little or no impact.

However, some other major companies refused to say whether their operations had been affected. "We can't get into details of our business operations," said Gap spokeswoman Claudia Hawkins.

When asked if Wells Fargo had experienced problems with the worm, spokeswoman Lucy Browne said: "I can't answer your question." Browne added that she was only authorized to say that none of its customer information had been compromised and that the bank was monitoring the infection.

Because of the silence, one expert said it is difficult to gauge the damage done by the worm.

"In general, they're going to keep it quiet when they have problems," said Lloyd Taylor, vice president of technology for Keynote Systems, a San Mateo firm that tracks Internet traffic. He said corporate network operators are likely to be embarrassed, particularly because Microsoft issued a security warning about the flaw more than a week ago.

"When the patch is out three weeks and you still get hit, who is going to want to explain that?" he asked.

So far, Taylor said the infection hasn't affected the overall flow of information on the Internet.

But Taylor believes that so many home computers have been affected that the Internet could be jammed on Saturday, when the worm is programmed to take over infected computers and use them, like zombies, to attack Microsoft's software update site.

Taylor suspects more than 1 million computers worldwide might be affected, several times the estimate from other security experts. He is particularly concerned that infected home computers in Australia, South Korea and Singapore could create traffic jams that could temporarily block or crash the transcontinental trunk lines that connect these regions to the North American Internet.

"It (Blaster) can probably take down the edges of the Net," Taylor said.

But for home users whose infected computers may be the sources of the expected attack, the experience is likely to be yet another annoyance.

"The attack will shut down any connectivity for your home," said Taylor.