Why do some incoming HTML messages look fine, but others are text

[Ursuline Guest House]

In Outlook 2003 with Vista Home Premium, what could possibly be causing some
incoming HTML messages to look fine, but others, from say HP or Home Depot,
to show up as raw HTML text?

Here are the message Internet headers:

Received: from hpccm-55.hpccm.postdirect.com ([206.165.246.55])
by isp.att.net (frfwmxc05) with ESMTP
id <20081117113518M05005p28le>; Mon, 17 Nov 2008 11:35:18 +0000
X-Originating-IP: [206.165.246.55]
DomainKey-Signature: a=rsa-sha1;
h=Date:From:Subject:To:X-Header-CompanyDBUserName:List-Unsubscribe:Reply-To:X-Header-MasterId:X-Header-Versions:Message-ID:MIME-Version:Content-Type;
b=rP3QdAUMYGgXz3hbXeTBw1B3hMcQkZFQIif62r5PlQiAKqoZiGeNZCe4cP1NiJzZmyz69R9ukiu/YFBK/0I+b6TdhNOLE1CVT05Lj/GSsFKV9x9sVwk8nu/n9xEVA0QX;
c=nofws; d=us.newsgram.hp.com; q=dns; s=yesmail1
Date: Sun, 16 Nov 2008 20:24:57 PST
From: Hewlett-Packard <[email protected]>
Subject: Special Edition Notebook - Exclusively at Circuit City
To: "*****" <[email protected]>
X-Header-CompanyDBUserName: hpccm
List-Unsubscribe:
<mailto:[email protected]?subject=unsubscribe>
Reply-To: (e-mail address removed)
X-Header-MasterId: 746254
X-Header-Versions: (e-mail address removed)
Message-ID: <[email protected]>
MIME-Version: 1.0
Content-Type: text/html; charset=us-ascii



Here's a snippet of the displayed body:

<http://link.p0.com/1x1.dyn?0QkHNUT9Bdrt4-PVRW2S=0>
» Projects
<http://link.p0.com/u.d?TmZa5ado1Uxgmqd=311&s=hpn_21462|746254|A5A6869B3
48BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
<http://images.postdirect.com/master/7/746254/spacer.gif> »
Online classes
<http://link.p0.com/u.d?AmZa5ado1Uxgmy=21&s=hpn_21462|746254|A5A6869B348
BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
<http://images.postdirect.com/master/7/746254/spacer.gif> »
Support
<http://link.p0.com/u.d?AmZa5ado1UxgmpN=151&s=hpn_21462|746254|A5A6869B3
48BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
<http://images.postdirect.com/master/7/746254/spacer.gif> »
Snapfish <http://link.p0.com/u.d?SmZa5ado1UxgmoS=41>
<http://images.postdirect.com/master/7/746254/spacer.gif> »
Printer supplies
<http://link.p0.com/u.d?FmZa5ado1Uxgmof=51&s=42904&hhosnl=hpn_21462|7462
54|A5A6869B348BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
<http://images.postdirect.com/master/7/746254/spacer.gif> »
Outlet
<http://link.p0.com/u.d?VmZa5ado1UxgmoY=61&s=42904&hhosnl=hpn_21462|7462
54|A5A6869B348BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
<http://images.postdirect.com/master/7/746254/spacer.gif> »
Online shopping
<http://link.p0.com/u.d?KmZa5ado1Uxgmol=71&s=42904&hhosnl=hpn_21462|7462
54|A5A6869B348BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
HP Pavilion dv5-1159se Special Edition Notebook PC- Click NOW to buy
exclusively at Circuit City!
<http://link.p0.com/u.d?TGZa5ado1Uxgmou=81&s=hpn_21462|746254|A5A6869B34
8BFE68|7AFBB66E25A68EAF0078BF1197AF3218>
HP Pavilion dv5-1159se Special Edition Notebook PC- Click NOW to buy
exclusively at Circuit City! <http://link.p0.com/u.d?LmZa5ado1Uxgmor=91>

Like I said, most HTML messages display just fine, but for the past several
months, messages from several larger corporations won't display properly.

All insight is appreciated! Thanks!