If you are using Word 2003 or Word 2007, you do this to prevent people from
editing the headers (and footers): In the Restrict Formatting and Editing
pane (Protect Document pane in Word 2003), click the check mark under
"Editing restrictions." Then choose the "No changes (Read only)" option.
Select the main body of the document and set it as an exception, by clicking
the appropriate user names under "Exceptions (optional)"; in this case you
would want to set an exception for "Everyone."

If you are using an older version of Word, see this article:
http://word.mvps.org/faqs/customization/ProtectWord97Header.htm.