Hello,
What version of SEP are you trying to install?
What OS are facing this issue on?
If the above steps does not work please try the following:-
1.Disable UAC.
2.If it's an upgrade uninstall SEP using cleanwipe utility and restart the system.
How to get the CleanWipe tool (Endpoint Removal)
OR
3. Manually uninstall SEP.
Delete the contents of the SEP from the computer and registry after backing up the registry, then try reinstalling SEP.
http://www.symantec.com/docs/TECH184988
Hope that helps!!