Many of the antivirus programs from Symantec, including most versions
in the Norton antivirus line, have to be uninstalled, not just disabled,
to stop them from occasionally creating new problems in Windows
Mail. I haven't heard if this also applies to a version they don't apply
the Norton name to, so you may have to uninstall it to find out.
Most versions in the McAfee antivirus family and a recent version in
the Trend antivirus family also cause assorted problems in Windows
Mail.
For a Norton antivirus product, you also need to run this cleanup
program to remove parts the uninstall leaves behind:
<
http://service1.symantec.com/SUPPORT/tsgeninfo.nsf/docid/2005033108162039>
You may even have to remove your email account under Windows
Mail, then create it again, for more cleanup.
If this solves your problem, you may want a replacement antivirus
program. I recommend either avast! or AVG, either one with a
custom install that tells it to leave out the part that scans email.
http://www.avast.com/eng/avast_4_home.html
http://free.grisoft.com/doc/5390/us/frt/0?prd=aff