not sure if this is what you mean. Using javascript, before you close the child form, you send the value to the parent form:
<script language="JavaScript" type="text/javascript">
function SendValue(form_name,combo_box,selected_index){
eval("window.opener.document."+form_name+"."+combo_box+".selectedIndex = '"+selected_index+"'");
window.close();
}
</script>
(you might wanna double check the 'selectedIndex' deal, i dont remember that very well)
When you open this child form, pass the form name and the name of the combobox you want to update in the link:
<SCRIPT language="Javascript">
function newuo(){
window.open('uonue.php?FORM_NAME=[the name of your form]&COMBO_BOX=[the name of your combobox]', 'frmuo', width=100, height=100);
}
</SCRIPT>
and receive the form_name in the child form to include it in your function SendValue(form_name,combo_box,selected_index)