To protect form fields in Microsoft Word 2007, you need to have enabled the Developer Tab. To enable this tab
follow the instructions in this post.
Once you have enabled the Developer Tab - select the Developer tab. In the Control section, click the button that looks like a folder, and that shows the tooltip "Legacy Tools". The Forms
fields are the first row of the dropdown on that button.

Once you have completed inserting the forms, checkboxes or dropdown fields, then select Protect Document -

A window will open to the right hand side of your word document. Now choose the level of protection to be applied:
Editing Restriction - Allow only filling in forms and then choose start enforcement
You will be asked to use a password which is handy