Did you follow the Steps to prepare computers to install Symantec Endpoint Protection Small Business Edition 12?
http://www.symantec.com/business/support/index?page=content&id=TECH91144&actp=search&viewlocale=en_US&searchid=1296850663389
Replacing SAV Corporate Edition with SEP (this is for SEP 11, but should apply to SEP12 as well)
http://www.symantec.com/business/support/index?page=content&id=TECH91144&actp=search&viewlocale=en_US&searchid=1296850663389