are you pushing the client to windows 7 or windows 8?
Just make sure that simple file share in enabled, check this prequisites here
Steps to prepare computers to install Symantec Endpoint Protection 12.1.x client
http://www.symantec.com/business/support/index?page=content&id=TECH163112
for slow wan check this
https://www-secure.symantec.com/connect/forums/deploying-sep-client-installation-package-over-wan