The reason for the failure of the installation would be last action performed before the rollback started and it would be found in the lines just above the result found for the keyword searched.

Example: In this example the installation failed as the start menu shortcut couldn't be created.
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2013-02-18T00:54:32.074Z INFO  I SIS    Executing action ( 294 ) - CreateService  currentPosition: 138348
2013-02-18T00:54:32.074Z DEBUG I SIS      [CreateServiceBase] Validated service name.
2013-02-18T00:54:32.074Z DEBUG I SIS      [CreateServiceBase] Validated service name.
2013-02-18T00:54:32.074Z DEBUG I SIS      [CreateServiceInternal] AlteredPath initialized to 'C:\Program Files\Symantec\Symantec Endpoint Protection\12.1.1101.401.105\Bin\snac.exe'.
2013-02-18T00:54:32.074Z INFO  I SIS      [CreateServiceInternal] Using ImagePath == 'C:\Program Files\Symantec\Symantec Endpoint Protection\12.1.1101.401.105\Bin\snac.exe'.
2013-02-18T00:54:32.074Z DEBUG I SIS      [CreateServiceInternal] ChangeServiceConfig2( SERVICE_CONFIG_FAILURE_ACTIONS, 'SNAC' ) succeeded.
2013-02-18T00:54:32.074Z DEBUG I SIS      [CreateServiceInternal] ChangeServiceConfig2( SERVICE_CONFIG_DESCRIPTION, 'SNAC' ) succeeded.
2013-02-18T00:54:32.074Z INFO  I SIS      [CreateServiceInternal] Created service 'SNAC'.
2013-02-18T00:54:32.074Z INFO  I SIS    
2013-02-18T00:54:32.074Z INFO  I SIS    Executing action ( 295 ) - CreateService  currentPosition: 139418
2013-02-18T00:54:32.074Z DEBUG I SIS      [CreateServiceBase] Validated service name.
2013-02-18T00:54:32.090Z DEBUG I SIS      [CreateServiceBase] Validated service name.
2013-02-18T00:54:32.090Z DEBUG I SIS      [CreateServiceInternal] AlteredPath initialized to '"C:\Program Files\Symantec\Symantec Endpoint Protection\12.1.1101.401.105\Bin\Smc.exe" /prefetch:1'.
2013-02-18T00:54:32.090Z INFO  I SIS      [CreateServiceInternal] Using ImagePath == '"C:\Program Files\Symantec\Symantec Endpoint Protection\12.1.1101.401.105\Bin\Smc.exe" /prefetch:1'.
2013-02-18T00:54:32.090Z DEBUG I SIS      [CreateServiceInternal] ChangeServiceConfig2( SERVICE_CONFIG_FAILURE_ACTIONS, 'SmcService' ) succeeded.
2013-02-18T00:54:32.090Z DEBUG I SIS      [CreateServiceInternal] ChangeServiceConfig2( SERVICE_CONFIG_DESCRIPTION, 'SmcService' ) succeeded.
2013-02-18T00:54:32.090Z INFO  I SIS      [CreateServiceInternal] Created service 'SmcService'.
2013-02-18T00:54:32.090Z INFO  I SIS    
2013-02-18T00:54:32.090Z INFO  I SIS    Executing action ( 296 ) - ShowRollbackMessage  currentPosition: 140534
2013-02-18T00:54:32.090Z INFO  I SIS      [ShowRollbackMessage] message = [Installing services]
2013-02-18T00:54:32.090Z INFO  I SIS    
2013-02-18T00:54:32.090Z INFO  I SIS    Executing action ( 297 ) - ShowInstallMessage  currentPosition: 140642
2013-02-18T00:54:32.090Z INFO  I SIS      [ShowInstallMessage] message = [Creating shortcuts]
2013-02-18T00:54:32.090Z INFO  I SIS    
2013-02-18T00:54:32.090Z INFO  I SIS    Executing action ( 298 ) - CreateShortcut  currentPosition: 140746
2013-02-18T00:54:32.090Z DEBUG I SIS      DeleteFileAction: target does not exist: C:\Documents and Settings\All Users\Start Menu\Programs\Symantec Endpoint Protection\Symantec Endpoint Protection.lnk
2013-02-18T00:54:32.090Z ERROR I SIS      Failed to acquire shell interface.
2013-02-18T00:54:32.090Z ERROR I SIS        
2013-02-18T00:54:32.090Z ERROR I SIS        This action failed to execute!  Dumping action paramters from the script:
2013-02-18T00:54:32.090Z ERROR I SIS          ShortcutPath=[C:\Documents and Settings\All Users\Start Menu\Programs\Symantec Endpoint Protection\Symantec Endpoint Protection.lnk]
2013-02-18T00:54:32.090Z ERROR I SIS          Target=[C:\Program Files\Symantec\Symantec Endpoint Protection\12.1.1101.401.105\Bin\SymCorpUI.exe]
2013-02-18T00:54:32.090Z ERROR I SIS          TargetDirectory=[]
2013-02-18T00:54:32.090Z ERROR I SIS          Arguments=[]
2013-02-18T00:54:32.090Z ERROR I SIS          IconFile=[]
2013-02-18T00:54:32.090Z ERROR I SIS          IconIndex=[0]
2013-02-18T00:54:32.090Z INFO  I SIS        ExecuteScript() - Successfully set failure event.
2013-02-18T00:54:32.090Z INFO  I SIS    ExecuteScript() returning ACTION_FAILED_WITH_ROLLBACK
2013-02-18T00:54:32.090Z INFO  I SIS  
2013-02-18T00:54:32.090Z INFO  I SIS  script completed with status: ACTION_FAILED_WITH_ROLLBACK
2013-02-18T00:54:32.106Z INFO  r SIS  TransitionToEnteringRollbackScript() - success
2013-02-18T00:54:32.106Z INFO  r SIS    
2013-02-18T00:54:32.106Z INFO  r SIS    starting execution of script: C:\Program Files\Symantec\Symantec Endpoint Protection\12.1.1101.401.105\Scripts\startrollback.sis
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The SIS_INST.LOG is located at the following location:
C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\12.1.XXXX\Data\Install\Logs

A few common errors:

1)
----------------------
Error in SIS_INST.log:
[StartService] StartService() failed for service 'BHDrvx86' with error 0x00000057.
Solution: http://www.symantec.com/docs/TECH181778
----------------------

2)
----------------------
Error in SIS_INST.log:
DeleteFileAction: target does not exist: C:\Documents and Settings\All Users\Start Menu\Programs\Symantec Endpoint Protection\Symantec Endpoint Protection.lnk
Failed to acquire shell interface.
Solution: http://www.symantec.com/docs/TECH170259
----------------------