Sending the same message more than once seldom gets you help any faster.
Unknown error and undeletable files in Windows Mail are usually due to
interference from an antivirus program, antispyware program, or antispam
program. What if any such programs do you have?
For a temporary fix, 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.
The Vista SP1 update includes this hotfix and more, but takes longer
to install.