Most versions of the McAfee antivirus program are known to occasionally
cause problems in Windows Mail.  Just disabling the McAfee program
seldom helps; you usually have to uninstall it instead.  You then often need
to run this cleanup program to take care of what the uninstall left behind:

<http://service.mcafee.com/FAQDocument.aspx?id=107083&lc=4105&partner=McAfee&type
=TS&ia=1>

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

If this solves your problem, you may want a replacement antivirus
program that doesn't cause problems in Windows Mail.  Most versions
of the Norton antivirus program and a recent version of the Trend
antivirus program are bad choices; they cause problems in Windows
Mail too.  I recommend either avast! or AVG, either one with a custom
install where you tell it to leave out the part that scans email.  If you
need or want to use a paid version instead, I've seen CA Anti-Virus
recommended.

http://www.avast.com/eng/avast_4_home.html

http://free.grisoft.com/ww.download?prd=afe

http://shop.ca.com/virus/antivirus.aspx