Hello,
This problem is fixed in the Latest version of Symantec Endpoint Protection 12.1 Release Update 3 (12.1 RU3) which has been released on June 6 2013.
New fixes and features in Symantec Endpoint Protection 12.1.3
http://www.symantec.com/docs/TECH206828
After applying Microsoft update KB2781197 to a Windows 8 computer, the Symantec Endpoint Protection client installer fails
Fix ID: 3123310
Solution: For installations on Windows 8, removed the option to enable/disable Windows Defender after install, as this is no longer supported. Windows Security Center (WSC) will automatically disable Windows Defender when Symantec Endpoint Protection registers with WSC.
For information on how to obtain the latest build of Symantec Endpoint Protection, read ‘Obtaining the latest version of Symantec Endpoint Protection or Symantec Network Access Control’
For more - Check these BLOG's:
1) Latest Symantec Endpoint Protection Released - SEP 12.1.RU3
https://www-secure.symantec.com/connect/blogs/latest-symantec-endpoint-protection-released-sep-121ru3
2) What's new with Latest Symantec Endpoint Protection SEP 12.1.RU3
https://www-secure.symantec.com/connect/blogs/whats-new-latest-symantec-endpoint-protection-sep-121ru3
3) Knowledgebase Articles for Symantec Endpoint Protection 12.1. RU3
https://www-secure.symantec.com/connect/blogs/knowledgebase-articles-symantec-endpoint-protection-121-ru3
4) Knowledgebase Articles for Symantec Endpoint Protection SBE 12.1. RU3
https://www-secure.symantec.com/connect/blogs/knowledgebase-articles-symantec-endpoint-protection-sbe-121-ru3
Hope that helps!!