Version: Word 2003 SP2

I'm trying to mailmerge information into a form which contains one combobox
form fieldand several text form fields. The protect document icon is greyed
out until I merge the document. When I do this the mail merge works fine and
I can select the protect document icon which activates the combobox but the
text form fields disappear on merging. Does anyone know how to maintain form
fields when merging a document?