Norton AntiVirus

Hello, I have Norton Internet Security.  I my computer is about 4 months old.  When I recieved the computer, this program was already installed.  I thought that this was all I had to do.  A couple of days ago when I went to run a scan, it said that my "virus definition" was expired.  The program told me to run a live update, I did so and it did not fix my problem.  I then downloaded Norton AntiVirus 2005 thinking that this was the program I am missing and it would fix the problem I am having.  I am afraid that my computer is not being protected.  If anyone knows what I have to do in order to fix this problem please let me know.  Also if there is a better AntiVirus/SpyWear/ any other protection please let me know.

Thank You

-Julianna