Microsoft today urged customers to upgrade from Windows 98 as it pulled support for the software.
The technology giant said the product was "outdated" and could expose users to security risks.
It has also ceased technical support for Windows 98 Second Edition and Windows Me.
Microsoft's website will still offer help and advice for customers who continue using the software.
The firm wants users to upgrade to a more secure system, such as Windows XP, as soon as possible.
A Microsoft spokesman said Windows 98 and Windows Me were developed before the era of "sophisticated" internet viruses.
"These products have reached the point of architectural obsolescence," he said.
"It would be irresponsible to convey a false sense of security by extending public support for this old product."
After today Microsoft will stop providing incident support options and security updates for Windows 98, Windows 98 Second Edition and Windows Me users.
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