If you're having trouble installing the Symantec Endpoint 11 client on a machine that was previously running Symantec AntiVirus server components you may have have to clean up the Windows Installer entries.
Links:
[1] http://www.techniquemicro.com/content/symantec-endpoint-client-installation-issue
[2] http://www.techniquemicro.com/tag/endpoint
[3] http://www.techniquemicro.com/tag/installation
[4] http://www.techniquemicro.com/security
[5] http://www.techniquemicro.com/software
[6] http://www.techniquemicro.com/tag/symantec