Two routes come to mind:
1. Try prssing Ctrl-Alt-Delete. In pre-XP systems this will bring up a list of running programs. Simply select the relevant program and click 'End Task'. In XP, you'll have to select 'Task Manager' first but you should then be able to close programs.
OR
2. Go to Start > Run. Type 'msconfig' (without the quote marks). Click 'Start'. Select the 'StartUp' tab. Click to remove the tick next to the Norton program. Click 'Apply', then 'OK'. Re-boot your PC. (It will start without loading Norton). Install your new software, then repeat the process (and reboot) to place a tick alongside Norton and get it loading at boot-up.
Chris