This came up in the Help system for Word 2000. (Go in to Help, and search for Protect and then look at the Protect Forms item.)
"Prevent users from changing a form containing form fields
After creating a form that users will view and complete in Word, you must protect it so that users can enter information only in the designated areas.
On the Tools menu, click Protect Document.
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 Password box. Users who don't know the password can still enter information in the form fields.
To protect the entire form, click OK.
To protect only parts of a form, those parts must be in separate sections. Click Sections, and then clear the check boxes of the sections you don't want to protect.
Tip When you are designing or changing a form, you can quickly protect the form or remove protection from it by clicking Protect Form on the Forms toolbar."
Hope this helps you.