Access 2000 Combo Box

I am trying to code a combo box with the event procedure recorded “On Not In List”. I have code that will work if the table I’m writing to doesn’t have a primary key. But if I try and use it on a table with a primary key, it will automatically add the auto number as one field each time I try to create a field. In other words I end up with the field I created and a field that is the next auto number. So. Hopefully someone encountered this problem and can shed some light on the frustration I’m suffering! Thx.