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 you have a 32-bit version
of Vista or a 64-bit version, try the hotfix for the 32-bit version first.
Certain common antivirus programs, such as most of those from
Norton or McAfee, tend to cause stuck messages 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.

The Vista SP1 update includes this hotfix, and enough more to make
it take hours longer to install.

http://support.microsoft.com/kb/936330

OklahomaJamie;706784 Wrote:

Hi Jamie,

This will help show you how to fix the stuck undeletable email message
problem in Windows Mail for all folders.

Installing the KB941090 update will just fix the problem for now. You
must stop the cause to prevent it from happening again. Read the section
titled "FIRST" in this tutorial about antivirus and anti-spyware/adware
programs to help prevent this from happening again, then install the
KB941090 Microsoft Update in the section titled "SECOND" to fix and be
able to delete the stuck message from the problem folder.

'Windows Mail problems'
(http://www.vistax64.com/tutorials/62560-windows-mail-problems.html)

Shawn