>How can I hide a password into a field
>in a .dbf field, so that no one can
>see it ?

You can't - you could store the information in an encrypted format, and encrypt/decrypt when you use it, but there's no field-level security mechanism, since a wide range of tools can open a .DBF. If you need to enforce security at the field level, you'd best look into a backend like SQL Server or Oracle.