I raised a ticket with Symantec and together we solved the problem.
The first step was to examine the file C:\Documents and Settings\All Users\Application Data\Symantec\Symantec Endpoint Protection\12.1.XXXX\Data\Install\Logs\SIS_INST.LOG
Where identified the following event:
2013-05-13T18:59:47.741Z INFO I SIS Executing action ( 2485 ) - CreateRegistryValue currentPosition: 1170366
2013-05-13T18:59:47.747Z INFO I SIS KEY_NAME=[SYSTEM\CurrentControlSet\services\WinDefend]
2013-05-13T18:59:47.747Z INFO I SIS VALUE_NAME=[Start]
2013-05-13T18:59:47.747Z DEBUG I SIS VALUE_TYPE=[DWORD]
2013-05-13T18:59:47.747Z DEBUG I SIS VALUE_DATA=[4]
2013-05-13T18:59:47.754Z ERROR I SIS ZwSetValueKey() failed in thal::RegistryWriteValue() status: 0xC0000022 = {Access Denied} A process has requested access to an object, but has not been granted those access rights.
2013-05-13T18:59:47.754Z ERROR I SIS RegistryWriteValue() failed. 0xC0000022 = {Access Denied} A process has requested access to an object, but has not been granted those access rights.
2013-05-13T18:59:47.754Z ERROR I SIS
2013-05-13T18:59:47.754Z ERROR I SIS Dumping action parameters from the script:
2013-05-13T18:59:47.754Z ERROR I SIS ValueType=[dword]
2013-05-13T18:59:47.754Z ERROR I SIS ValueData=[4]
2013-05-13T18:59:47.754Z ERROR I SIS ValueName=[Start]
2013-05-13T18:59:47.754Z ERROR I SIS Exists=[Overwrite]
2013-05-13T18:59:47.754Z ERROR I SIS WOW64=[Default]
2013-05-13T18:59:47.754Z ERROR I SIS Key=[HKLM\SYSTEM\CurrentControlSet\services\WinDefend]
2013-05-13T18:59:47.754Z INFO I SIS ExecuteScript() - Successfully set failure event.
2013-05-13T18:59:47.755Z INFO I SIS ExecuteScript() returning ACTION_FAILED_WITH_ROLLBACK
Showing that there was a problem disabling Windows Defender.
Further trouble shooting showed that Windows Defender was corrupt and couldn't start. I resolved this by uninstalling Microsft Security Essentials. Once Windows Defender was able to start again, I was able to install Symantec (which disables Windows Defender).