NOTE: If you ever feel compelled to send me an e-mail, always use plain text mode!
Microsoft Knowledge Base Article 883257 contains the following summary and introduction:
SUMMARYIn Microsoft Windows XP, to help preserve the user's privacy and prevent virus attacks, Microsoft Outlook Express 6 blocks e-mail attachments that are considered unsafe and HTML content such as images. In Microsoft Windows XP Service Pack 2 (SP2), to prevent malicious scripts from running automatically, plain text mode is also turned on in Outlook Express.
INTRODUCTIONThis article describes plain text mode in Outlook Express and answers the following questions:
What is plain text mode?
Why use plain text mode?
What features are not available in plain text mode?
How do I use plain text mode in Outlook Express?
How do I switch between reading or composing e-mail messages as plain text and HTML easily when I want to?