In some cases you may want to disable HTML rendering and editing of
email messages while not using Microsoft Word as your editor.

Microsoft has documented a workaround for Office XP which also applies
to Office 2003

http://support.microsoft.com/kb/307594

You will need to open the registry editor and set the dword value
"ReadAsPlain"=dword:00000001 under the following Key

HKCU\\Software\\Microsoft\\Office\\11.0\\Outlook\\Options\\Mail