The havoc caused by the MSBlaster virus could soon happen all over again.
MSBlaster spread wide and caused huge disruption
Microsoft's security patch to thwart MSBlaster leaves some versions of Windows still vulnerable to infection by similar web worms.
The MSBlaster virus infected thousands of machines in August and swamped net links with data as it looked for vulnerable machines to infect.
Microsoft's patch has been updated to close the new holes but experts expect to see viruses exploiting them soon.
The new "critical" security vulnerabilities discovered by Microsoft revolve around the same Windows feature that MSBlaster or LovSan worm so successfully exploited.
The susceptible feature helps programs work together across a network.
The patch issued by Microsoft to protect machines against MSBlaster shut off some, but not all, of the deficiencies in this feature.
Now Microsoft is warning that viruses that work in a similar way to MSBlaster could slip through these holes and cause an outbreak on a similar scale to last month's worm.
So far malicious hackers do not seem to be targeting the newly found vulnerabilities, said Jeff Jones, senior director of Trustworthy Computing at Microsoft.
Windows NT Workstation 4.0
Windows NT Server 4.0
Windows NT Server 4.0 (Terminal Server Edition)
Windows Server 2003
"We have a worry that history has shown us there are malicious individuals out there that could create an attack of some sort against it," said Mr Jones.
But security experts expect that viruses tuned to look for these new holes will soon appear.
MSBlaster appeared barely a month after Microsoft warned about the deficiencies that the virus exploited.
Because the new vulnerabilities affect several different versions of Windows, experts believe any outbreak could be serious.
Machines running Windows 95, 98 and ME are not at risk from these new vulnerabilities.
Microsoft urged people to download and apply the latest patch, to keep anti-virus software up to date and use a firewall to protect against unwanted intrusions.