Download and install this hotfix, with Windows Mail not running:
http://support.microsoft.com/default.aspx/kb/941090
Although it's labelled as for mail stuck in the Outbox, it also works in
other folders. If you don't know whether to use the 32-bit version or
the 64-bit version, try the 32-bit version first. Certain common
antivirus programs, such as most of those from Norton or McAfee,
tend to cause this and other problems in Windows Mail, although
seldom immediately, so if you have such a program, remember
where to find the hotfix, and hope it will install again if you need it
again. A recent version of the Trend antivirus program also causes
assorted problems in Windows Mail.