Scan the computer in safe mode using the previous version of Symantec installed in your PC or AVG Anti-virus or any reliable antivirus tool from the web remove the viruses if any. Make sure that your PC is free of virus before installing SEP; otherwise you will end up with post installation problems and difficulties with updates and system performance.
(Last resort you can take back-up of important data and format & reinstall OS on your system and then do the antivirus installation.)
Uninstall the previous version of installed Symantec (if any) through Control Panel including other antivirus tools if any.
Install Symantec Endpoint Protection new version.