forget,
I had exactly the same prob as you (on Win XP). Liveupdate set to work, but never seemed to get the definitions with me telling it to. I found this article on Norton's site. Seems there's a problem with the task scheduler and LU. Step one on the following page sorted it for me:
http://service1.symantec.com/SUPPORT...&osv=&osv_lvl=
Before you apply the fix, go "Start/All Programs/Accesories/System Tools/Task Scheduler" and see when Symantec Net Detect last ran............do the same after you apply the fix to check it has worked.
Hope this helps.
Osbo