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Support for Windows 2000 is no longer available from its manufacturer. Microsoft's Mainstream Support End Date for Windows 2000 Server and Windows 2000 Professional was June 30, 2005. The Extended Support End date was July 13, 2010
While SEP 12.1 cannot be installed on this unsupported Windows 2000 OS, Symantec Endpoint Protection 11 can try to protect Windows 2000 computers until they have been retired from the organization.
Reference: About Support for Microsoft Windows 2000 in Symantec Endpoint Protection 12.1
It's time to upgrade to the latest operating system because depending on the nature of the threats involved, Windows Server OS may be susceptible to viruses and risks that more recent iterations of Windows is immune to