Why doesn't Norton stop the FBI Moneypak virus from infecting my computer in the first place?

This is the fourth time I have gotten this virus and this time I am having a hard time removing it.  I can't seem to get to Safe Mode, as soon as it gets there, it logs off and restarts.  I even tried running my Norton bootable recovery disc, but it didn't find the virus.  I have the Power Eraser on my computer already, but I can't get to it.  I think once, I was able to use System Restore, but I don't remember how I got there.  I've been trying for 1 1/2 days to fix this, but no dice.  Please help me out here.