Force plain text

How can I force Outlook to show all my emails in Plain Text format?

This is possible since Outlook 2002/XP. From there on, the way to do this differed with each Outlook version.

Outlook 2007
Tools-> Trust Center…-> E-mail Security-> Read all standard mail in plain text

Outlook 2003
Tools-> Options…-> button Email Options…-> Read all standard mail in plain text

Outlook 2002/XP
In this version it is only a registry option. There is no native way to do this via the Outlook interface.

Key: HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Options\Mail
Value name: ReadAsPlain
Value Type: REG_DWORD
Value: 1

When this key is set to 0 or missing, all messages will display in their normal message format.

Note 1: You can also set this option via OutlookTools (free).
Note 2: If you are running a virus scanner that integrates with Outlook, you might also find an option there to convert all messages to Plain Text. Note that in most cases, this option works separate from the option set in Outlook. See the documentation of your security suite for more information.
Note 3: Administrators can also set this as a policy.