Do you have a lot of .tmp, .err and .dat files which have the "date modified" value as a few or more days old in the subfolders of "..Program Files\Symantec\Symantec Endpoint Protection Manager\data\inbox\" ? (Kindly check on all the subfolders and their subfolders as well)
If no, then running LUALL in "interactive" mode would indicate at which step the update fails:
Control Panel>Symantec LiveUpdate>Select "Interactive mode">Save.
Then, Start>run>LUALL
First Liveupdate would contact the Server, then check for required updates, then show a list of updates which it would download, then it would start the download after which the installation would occur.