Home computer users have been hit by an Internet "worm" called Blaster, even as several experts said a new variant had emerged.
A new strain of the malicious program was said to be working its way through cyberspace on Wednesday, but it was considered no more harmful than the original, which can cause a computer to crash and infect other computers on the Web.
The worm, also called MSBlaster or LoveSan, infected office and home computers in the United States on Monday and quickly spread around the world, taking advantage of a security hole discovered last month in Microsoft's Windows 2000, Windows XP, Windows NT and Windows Server 2003 operating systems.
At least 230,000 computers were infected by Blaster, according to a sample by anti-virus vendor Symantec, while Moscow-based anti-virus provider Kaspersky Labs put the number closer to 300,000.
Patches for the hole, except for Windows NT 4.0, which the company no longer supports, were put online by Microsoft at its Web site (http://www.microsoft.com).
By yesterday, most US companies with expert technicians were able to stamp out the worm, but home users found it difficult to find and apply the patch.
Microsoft has said Blaster crashes some systems and infects others but otherwise does no damage. Although some corporate networks were slowed by the worm, no impact on overall Internet traffic was detected.
Experts warned, however, that more virulent "copy-cat" worms could be on the horizon.
"There is definitely another, more effective worm code sitting on researchers' systems out there," said Mr Tom Parker, director of research at Pentest, a British-based security consultancy. "Whether or not it will be released is now in question due to the vulnerability being all wormed out".
Even though the worm continued spreading on Wednesday, the number of new infections appeared to have gone down by half from the previous day, said Mr Alfred Huger, a senior director of engineering at Symantec.