Here is a quick tutorial that can fix that issue:
http://www.vistax64.com/tutorials/62560-windows-mail-problems.html.
Thanks go to Brink from Vistax64 for the tutorial!

Signature
Dustin Harper
dharper@vistarip.com
http://www.vistarip.com | Vista Resource & Information Page
>I am trying to delete an incoming message from my inbox and keep getting an
> error message. The body of the message says: Windows Mail encountered an
> unexpected problem while displaying this message. Check your computer for
> low memory or low disk space and try again.
>
> All this has been tried and I still can't get the message deleted.