http://email.about.com/cs/outlooktips/qt/et041401.htm
Use Word as Your Default Email Editor in Outlook
To set Word as the default editor for email messages in Outlook:
Select Tools | Options from the menu in Outlook.
Click on the Mail Format tab.
Make sure the Use Microsoft Office Word 2003 to edit e-mail messages checkbox is selected.
--In versions before Outlook 2003, the text reads Use Microsoft Word to edit e-mail messages.
Close the dialog.