Sometimes Liveupdate can get confused.
Are you positive that you have got a virus free machine, you can check by going to the Symantec site and doing an online scan.
Provided your machine is clean, you can often get round the problem by re-downloading the file that would not install.
Before you can do this you have to remove the affected file which is already stored on your machine in the "Symantec shared folder" Usually there are a series of folders under Virus Defs which have a date in the title.
Hope this helps