A recent version of the Trend software (details not available) causes
assorted problems in Windows Mail. So do most versions from
Norton or McAfee. These all need to be uninstalled, not just
disabled, to stop them from creating new problems, although they
are slow at creating problems.
Once you do this, you may have to delete your email account under
Windows Mail, then create it again, to finish the cleanup.
If this fixes 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