In one of the antivirus program families from Symantec, the Norton
family, most versions cause assorted problems with Windows Mail
and must be uninstalled, not just disabled, to stop them from
creating new problems occasionally. I don't know if they copied
the part causing these problems to any other antivirus family they
have; you may consider uninstalling it to find out.
If they did, you should also run this cleanup program:
<
http://service1.symantec.com/SUPPORT/tsgeninfo.nsf/docid/2005033108162039>
You may even need to remove your email account under Windows
Mail, then restart Windows Mail and create the account again for
more cleanup.
In this case, let us know if that fixes the problem, so we can tell
other people using the new Symantec antivirus program.