If you don’t like HTML mail or stationery, you can force plain text as the formatting for all incoming email.

In Outlook 2007, its in Tools, Trust Center, Email security. Outlook 2003 has it in Tools, Options, Email options. Both Outlook 2007 and 2003 allow you to change the format of any message back to HTML by clicking on the Infobar. All replies will be plain text.

The only other version that supports ‘read as plain text’ is Outlook 2002 SR1 update (does not work on earlier builds), you need to use a registry key:
HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Options\Mail
Add a DWORD of READASPLAIN, value of 1