Norton and windows firewalls definately conflict, numerous thread from people with this problem.
Norton is the better one of the two so switch off XP DO NOT RUN WITHOUT ANY.
If Norton causes problems then save your money a good combiaantion is:
ZoneAlarm Firewall click here
AVG antivirus click here
Anti Spyware :-
Adaware click here
Spybot S&D click here
Spywareblaster click here