Use the combo box wizard to create your combo box, and then only select the column(s) you actually want to be displayed. Access will automatically pull and use the PK of the table in the cbo.
Alternatively, make sure the query behind the combo box is pulling the columns you want, then make sure the column count has the same number of columns, and that they are displayed properly.
Column Count: 4
Bound Column: 1
Column Width: 0";1";1";1"
This will display the last three and not the PK, and be bound to the PK.
Does this help?