Ok, so I've tried the installation again, and since "return value 3" shows twice in the log, I'm posting the lines before each ocurrence:
MSI (s) (30:90) [19:08:23:765]: Added new source 'C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\12.1.1000.157.105\Data\Cached Installs\' with index '2'
MSI (s) (30:CC) [19:08:23:781]: Executing op: ActionStart(Name=StartServices,Description=Starting services,Template=Service: [1])
AddCacheAsInstallSource Successfully added C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\12.1.1000.157.105\Data\Cached Installs\ as source dir for {FA689023-0B72-4771-98A6-A1C927E58207}
MSI (s) (30:CC) [19:08:23:781]: Executing op: ProgressTotal(Total=1,Type=1,ByteEquivalent=1300000)
MSI (s) (30:CC) [19:08:23:781]: Executing op: ServiceControl(,Name=SepMasterService,Action=1,Wait=1,)
MSI (s) (30:CC) [19:08:26:250]: Executing op: ActionStart(Name=ShowServiceProgress_RB,Description=Executing rollback script via service,Template=[1])
MSI (s) (30:CC) [19:08:26:250]: Executing op: CustomActionSchedule(Action=ShowServiceProgress_RB,ActionType=3329,Source=BinaryData,Target=ShowServiceProgress_RB,CustomActionData={FCF84532-5E19-4602-85DC-07AD26FE0070};SOFTWARE\Symantec\Symantec Endpoint Protection;Executing rollback script via service;)
MSI (s) (30:CC) [19:08:26:265]: Executing op: ActionStart(Name=ShowServiceProgress,Description=Executing install script via service,Template=[1])
MSI (s) (30:CC) [19:08:26:265]: Executing op: CustomActionSchedule(Action=ShowServiceProgress,ActionType=3073,Source=BinaryData,Target=ShowServiceProgress,CustomActionData={FCF84532-5E19-4602-85DC-07AD26FE0070};SOFTWARE\Symantec\Symantec Endpoint Protection;Executing install script via service;)
MSI (s) (30:34) [19:08:26:296]: Invoking remote custom action. DLL: C:\WINDOWS\Installer\MSID0.tmp, Entrypoint: ShowServiceProgress
ScriptGen: ShowServiceProgress() MSIRUNMODE_SCHEDULED
ScriptGen: ShowServiceProgress() calling WaitForSingleObject(scriptStarted) ...
ScriptGen: ShowServiceProgress() WaitForSingleObject(scriptStarted) returned WAIT_OBJECT_0
ScriptGen: ShowServiceProgress() script execution failed.
ScriptGen: ShowServiceProgress() reset script failure event.
ScriptGen: ShowServiceProgress() is returning an error (so close to the end!)
MSI (s) (30:CC) [19:11:45:125]: User policy value 'DisableRollback' is 0
MSI (s) (30:CC) [19:11:45:125]: Machine policy value 'DisableRollback' is 0
Action ended 19:11:45: InstallFinalize. Return value 3.
and
MSI (s) (30:CC) [19:12:52:296]: Executing op: CustomActionRollback(Action=installFailure,ActionType=1281,Source=BinaryData,Target=installFailure,)
MSI (s) (30:44) [19:12:52:312]: Invoking remote custom action. DLL: C:\WINDOWS\Installer\MSIEB.tmp, Entrypoint: installFailure
IDC::CancelEvent Signaling Global\SymIDCInstallComplete
IDC::CancelEvent Received WAIT_COMPLETE_EVENT event
IDCCA: Failed to set property IdcXmlPath to C:\WINDOWS\TEMP\idc.xml (6)
Action ended 19:12:52: waitForIDCEvent. Return value 0.
IDCCA: Machine ID: a62c165621377cf9c164aaca8e79a82f0433d16
IDCCA: HttpQueryInfo: 200 OK Error: 0
IDCCA: SETI::CUploadFile::logout200 OK
MSI (s) (30:CC) [19:13:02:187]: Executing op: ActionStart(Name=CommunicateRollback,,)
MSI (s) (30:CC) [19:13:02:187]: Executing op: CustomActionRollback(Action=CommunicateRollback,ActionType=1345,Source=BinaryData,Target=CommunicateRollback,CustomActionData=12.1.1000.157;C:\TEMP\Clt-Inst\;;105)
MSI (s) (30:5C) [19:13:02:218]: Invoking remote custom action. DLL: C:\WINDOWS\Installer\MSIEC.tmp, Entrypoint: CommunicateRollback
Communicate RB: calling communicate state with the following arguments:
Communicate RB: Prodversion = 12.1.1000.157
Communicate RB: PathToSylink = C:\TEMP\Clt-Inst\
Communicate RB: Oldversion =
Communicate RB: ReasonStr =
MSI (s) (30:CC) [19:13:03:812]: Executing op: End(Checksum=0,ProgressTotalHDWord=0,ProgressTotalLDWord=0)
MSI (s) (30:CC) [19:13:03:812]: Error in rollback skipped. Return: 5
Communicate RB: StatusCode = 302469120
MSI (s) (30:CC) [19:13:03:828]: No System Restore sequence number for this installation.
MSI (s) (30:CC) [19:13:03:828]: Unlocking Server
MSI (s) (30:CC) [19:13:03:859]: PROPERTY CHANGE: Deleting UpdateStarted property. Its current value is '1'.
Action ended 19:13:03: INSTALL. Return value 3.
Again, thank you all for your help.
Cheers!