<script language="javascript" type="text/javascript">
function fillcheckbox(selection)
{
document.form1.mycheckbox.value = selection.value;
}
</script>
<select name="combo" onchange="fillcheckbox(this);">
.....
</select>
<input type="checkbox" name="mycheckbox" value="zero">
....
Remember that the value of the checkbox will only be passed to the form's action page when the checkbox is selected.