You may have decoding for the picture files and other attachments
turned off. Click on Tools, then Options, then Read. If there is a
check mark in the box before Read all messages in plain text,
click on this box, then Apply. Click on OK. Restart Windows
Mail.
If this doesn't help, you may be getting attachments with an
encoding for which Vista doesn't include a decoder. If so, copy
about the first page to such a message into your next request
for help for this.