Changing the permissions, per the above link, did the trick. Thanks
'HKEY_CURRENT_USER\SOFTWARE\Symantec\Symantec Endpoint Protection' - changed to full control
'HKEY_CURRENT_USER\SOFTWARE\Symantec\Symantec Endpoint Protection\AV\Custom Tasks' - did not have to change (already set to full)
Tried turning UAC back on but received a different set of circumstances - clicked scan, nothing happens, clicked scan again and says a scan is already running. Re-booted, re-tested, same result. Turned UAC off and all is fine.