Setting Query Criteria To Be 'blank' Depending On The Criteria Of A Combo Box

I have set up a database that stores actions (i.e jobs). In the table; two of the fields are...'required completion date' and 'actual completion date'. I wish to lookup, by using a query, all of the open actions (those which havent yet been complete (i.e the 'actual completion date' is null)) and then later on all those which are overdue (i.e the 'actual completion date' is null And the 'required completion date' <today....this being the criteria for an overdue action).

However, I have used a form which has a combo box which contains the values open and overdue. When a selection has been made I want a form to display with the results depending on the selection that has been made. I am capable of creating a form based on a query, but am unsure of how to construct the query with the correct criteria based on the option that is selected from the form.

Any help would gratefully be appreciated. Thanks