Mal, I have no heartening news for you. I installed Nortons Firewall about 6 months ago and that was even after I'd read complaint after complaint on the net from users whose systems kept crashing or freezing thanks to the firewall. You are NOT alone. Mine had only been installed 5 minutes before everything seized and I thought 'here goes....' however, after a couple of reinstalls it seemed to have settled. If you do a quick user search for that particular firewall I think you'll find that many people have been unable to continue using it because it seems to be incompatible with certain elements of their setup or programs already installed. I can't speak for XP but then again, that is ALSO notorious for being 'sensitive' to certain types of software.
I now have ZoneAlarmsPro. So far so good. Excellent in fact

Totally stealthed, reliable and no incompatiblities. For now

I'm not sure you or symantec will find anything specific that's contributing to your problem. It seems to be quite a common issue. Sorry mate.
Keep at it and let us know if you get anything more postitive from symantec. I for one would like to know what their answer is.
Good luck