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Symantec Endpoint Protection 12.1 RU2 and later are supported on Windows 8 and Windows Server 2012.
But Windows 8.1 is only supported with Symantec Endpoint Protection 12.1 RU4 and later.
In future refer landing page to find Fix notes, Releatse notes & System requirements info:
http://www.symantec.com/docs/TECH163829