Most versions of the McAfee antivirus program tend to cause assorted
problems in Windows Mail, although seldom immediately. Just disabling
the McAfee program seldom helps; you usually have to uninstall it
instead to get much improvement. You often then 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 even need to remove your email account under Windows
Mail, then restart Windows Mail and add the account again.
If this cures your problem, you may want a replacement antivirus
program that doesn't cause such problems. 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
Most versions of the Norton antivirus program and a recent
version of the Trend antivirus program are bad choices as
replacements, since they also tend to cause problems in Windows
Mail.