Hi ya! I have had this problem myself a while ago, it is quiet easy to fix (well it was for me) hopefully for you too, what it is the virus definitions have became currupted.
1, make sure that the auto protect is turned off
2, start windows explorer, and navigate to the following folder:C:\programs\common files\symantec shared\virusDefs
3,in the right pane, locate the numbered folders.They will look something like this 20000713.068 20010118.008 etc
4,Delete each one.DO NOT Delete any of the other folders!!
5,close windows explorer
6,click start,point to search,and then click files or folders
7,make sure that look is set to (C
8,in the "search for files or folders named" box type or copy and paste the following, and then click search now
livetri.zip settings.liveupdate
9,Delete any files that are found!!
10, now run liveUpdate to download new virus definitions
if you follow the steps you should have norton up and running again!