These are the exact steps for protecting selected sections in Word 2003:

 1. On the Tools menu, click Protect Document.
 2. In the Protect Document task pane, under Editing restrictions,
select the Allow only this type of editing in the document check box,
and then click Filling in forms in the list of editing restrictions.
 3. To add protection to only parts of a form, click Select sections,
and then clear the check boxes for the sections you don't want to add
protection to.
    Note  To add protection to only parts of a form, those parts must
          be in separate sections. (On the Insert menu, click Break
          to create section breaks.)

 4. Click Yes, Start Enforcing Protection.
 5. To assign a password to the form so that users who know the
password can remove the protection and change the form, type a password
in the Enter new password (optional) box, and then confirm the password.
Users who don't know the password can still enter information in the
form fields.

From the article at
http://office.microsoft.com/en-us/word/HP052302701033.aspx?pid=CH063563531033
(Create forms that users complete in Word)