Windows Mail is sensitive to interference from certain antivirus
programs. Therefore, email scanning in any antivirus must be
disabled, for reasons explained here:
http://thundercloud.net/infoave/tutorials/email-scanning/index.htm
If turning off the email scanning function does not resolve your problem,
do a trial uninstall of your antivirus. If no improvement, you can
always reinstall it. Instead of uninstalling your antivirus, consider
upgrading to Windows Live Mail, because it is less prone to
suffering bad effects from overly aggressive antivirus programs.
http://download.live.com/wlmail
Note that Microsoft has essentially abandoned Windows Mail
since about July 2006, when they switched development to
Windows Live Mail .