norton starts a lot of services, and takes a lot of time to shut them off. ya, u could switch over to avast or avg which i think are anyway more effective !
it would be better if you shut off some unnecessary xp services, as services take time to shutoff.
also make sure you are not clearing the page file at every shut off. this consumes time.
control your start-ups, and remove those which are not really required by you.
use tune-up utilities. its good and will help you :
www.tune-up.com
also visit :
http://www.thinkdigit.com/forum/viewtopic.php?t=17845
it tells you how to make xp start/shutdown faster.
i have winxp sp2 on a 128mb ram. try the above, and you will find your pc shutting down faster.