Help Changing Value of Textbox Based on a Selection in a ComboBox

I have a form (Access2000) with two controls: One ComboBox and one TextBox. ComboBox shows field "MM_filename" and lists its contents in the combobox. TextBox is "supposed" to show field "MM_id" (which is the "primary key". How can I do to the TextBox to change its value to the ID of the selected filename in the ComboBox??

thanks