In most cases, that is a good fix, although nothing is 100% effective.
Norton Antivirus 2008 (and earlier) is among the top three troublemakers
for Windows Mail. Symptoms often don't surface until several months have
gone by, or until an update occurs.
First, be sure to disable its email scanning, for the reason explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
Several users here have reported that Symantec will give you a free
upgrade to Norton Antivirus 2009, which cures the problem.
In a worst case scenario, you may have to uninstall Norton, followed by
using their debris removal tool:
http://service1.symantec.com/Support/tsgeninfo.nsf/docid/2005033108162039
Instead of uninstalling your antivirus, another option is to upgrade to
Windows Live Mail, which is more resistant to the adverse effects of overly
intrusive antivirus products:
http://get.live.com/wlmail/overview