Hi Elizabeth, welcome to the forums.
You will have to uninstall Norton if its the NAV 2003 or 2004, they don't mix very well. It can cause a conflict having two AV programs running in XP, NT or W2K at the same time.
Norton can also be a bit of a pig to get rid of, leaving behind remnants that can still interfere with the installation of avast.
I would suggest that you download RejZor's avast!
External Control program, this has a tool to remove the remnants of NAV 2004 after an uninstall. There are also a number of other useful tools to interface with avast.
HTH David