Neither an antivirus program nor a firewall will prevent installation of
software which you agree to--perhaps as part of a free game or other
software, or as part of the operation of a peer to peer sharing system.
I would recommend temporarily disabling as much of the Norton protective
software as you can--real-time antivirus scanning and the firewall. Unplug
the Internet connection first.
I believe you can turn off most of the parts of their recent apps via the
main panel, and that's what I'd do for long enough to attempt another
install. Basically, shut down as much third-party software as you can,
disconnect from the Internet, and try the install again.