Two possible approaches.
1) If you have a relatively small number of fields, base your combo on a query that returns all of the fields you want in the form. Make the column width zero for every column except your PK. Add text boxes to your form for each feild and set the control source to be the relevant column number from the combo.
2) For a larger number of fields, base the combo on a query that returns the PK only. Create a second query that returns the other fields that you want on the form and bind the form controls to that query. Use the value in the combo box as a criterion for the query ans use the After Update event of the combo to refresh the form when the selected PK changes.