When it fails it should give you an error code. click on the 'more info' link and it will take you to the Symantec support site and list some likely causes. The suggestion I most often see when this happens is to try again in a few hours or the following day.
I have my NAV2004 set to update my virus definitions automatically but if I decide to do it manually I click on "Live Update". Does anyone know if this updates my virus definitions or is it updating the various NAV components?