I'm not running Norton anymore because it is such a resource hog, but if you go to the control panel, there are options to scan your email. If these are set, check your outlook express settings. I'm not sure if XP SP2 changed anything in there with all of the security changes because I'm using Outlook.
If you still have problems, check out AVG (
www.grisoft.com). Its working GREAT for me and my family.