Is it really a userform (a custom dialog into which data is entered and then
transferred into a document) or a document using formfields which allow the
user the enter the information directly into the document.  For the latter,
the document must be protected for the formfields to work and that is done
either by using the icon on the Forms toolbar (View>Toolbars) or by
selecting Protect from the Tools menu.

If it is a userform, use the .Protect command in the code behind the form.

See the article "How to create a Userform" at:

http://word.mvps.org/FAQs/Userforms/CreateAUserForm.htm

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Part 1: Create professional looking forms in Word
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Part 2: Adding Automation to your Word forms.
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Part 3: Learn more VBA (macros) to automate your forms.
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Part 4: Use custom dialog boxes in your Word forms
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Part 5: Connect your AutoForm to a database to save input time and keep
better records!
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