Hi,
Thank you for posting in Symantec community.
I would be glad to answe your query.
There are three main reasons caused this issue:
1) Windows Defender
2) Microsoft hotfix KB2781197 installed.
3) Install account has privilege issue, some XP upgrades to Windows 8 will have this type of problem.
Reference: Troubleshooting for failed install of SEP 12.1 RU2 client on Windows 8 (Win8)
http://www.symantec.com/docs/TECH206760
As you mentioned it was already installed it might be possible it was not removed 100%.
Run the cleanwipe tool and then try to install it again.
https://www-secure.symantec.com/connect/forums/how-get-cleanwipe-tool-endpoint-removal
If suggested steps didn't work then try to install the latest version of SEP (SEP 12.1 RU3).
You might be aware SEP 12.1 RU3 has a following fix.
After applying Microsoft update KB2781197 to a Windows 8 computer, the Symantec Endpoint Protection client installer fails
Fix ID: 3123310
Symptom: The following lines are from the SEP_INST.log generated from a failed installation attempt:
(SERVER) MSI (s) (04:18) [08:56:49:093]: Executing op: CustomActionSchedule(Action=ShowServiceProgress,ActionType=3073,Source=BinaryData,Target=ShowServiceProgress,CustomActionData={3771A34D-2132-48EA-A486-D62ECDF9D553};SOFTWARE\Symantec\Symantec Endpoint Protection;Executing install script via service;)
(SERVER) MSI (s) (04:5C) [08:56:49:250]: Invoking remote custom action. DLL: C:\WINDOWS\Installer\MSI6B0.tmp, Entrypoint: ShowServiceProgress
(UNKNOWN) ScriptGen: ShowServiceProgress() MSIRUNMODE_SCHEDULED
(UNKNOWN) ScriptGen: ShowServiceProgress() calling WaitForSingleObject(scriptStarted) ...
(UNKNOWN) ScriptGen: ShowServiceProgress() WaitForSingleObject(scriptStarted) returned WAIT_OBJECT_0
(UNKNOWN) ScriptGen: ShowServiceProgress() script execution failed.
(UNKNOWN) ScriptGen: ShowServiceProgress() reset script failure event.
(UNKNOWN) ScriptGen: ShowServiceProgress() is returning an error
(UNKNOWN) CustomAction ShowServiceProgress returned actual error code 1603
Solution: For installations on Windows 8, removed the option to enable/disable Windows Defender after install, as this is no longer supported. Windows Security Center (WSC) will automatically disable Windows Defender when Symantec Endpoint Protection registers with WSC
Reference: New fixes and features in Symantec Endpoint Protection 12.1.3
http://www.symantec.com/docs/TECH206828