Most versions of the Norton program are incompatible.  So are most
versions of the McAfee program and a recent version of the Trend
program.  For each of these programs, just disabling them seldom
helps; you usually have to uninstall them instead.  For the Norton,
you often then need to run this cleanup program, to take care of what
the uninstall left behind:

<http://service1.symantec.com/SUPPORT/tsgeninfo.nsf/docid/2005033108162039>

You may even have to remove your email account under Windows
Mail, then restart Windows Mail and add the account again.

Then you should click on the Outbox and each of the messages in it.
If any of them show an error message instead of a message you sent,
move them to the Drafts folder or delete them.  If any refuse to move
or delete, download and install this hotfix, with Windows Mail not
running:

http://support.microsoft.com/default.aspx/kb/941090

Although it's labelled as for mail stuck in the Outbox, it also works in
other folders.  If you don't know whether you have a 32-bit version
of Vista or a 64-bit version, try the hotfix for the 32-bit version first.
The Vista SP1 update includes this hotfix and more, but takes longer
to install.

Yes.  :-(