The check box is not in admin.php, it is in admin/orders.php on line 611 (approx), you need to replace (assuming you are using the latest zencart;)):
<td class="main"><strong><?php echo ENTRY_NOTIFY_CUSTOMER; ?></strong> [<?php echo zen_draw_radio_field('notify', '1', true) . '-' . TEXT_EMAIL . ' ' . zen_draw_radio_field('notify', '0', FALSE) . '-' . TEXT_NOEMAIL . ' ' . zen_draw_radio_field('notify', '-1', FALSE) . '-' . TEXT_HIDE; ?>] </td>
with
<td class="main"><strong><?php echo ENTRY_NOTIFY_CUSTOMER; ?></strong> [<?php echo zen_draw_radio_field('notify', '1', FALSE) . '-' . TEXT_EMAIL . ' ' . zen_draw_radio_field('notify', '0', true) . '-' . TEXT_NOEMAIL . ' ' . zen_draw_radio_field('notify', '-1', FALSE) . '-' . TEXT_HIDE; ?>] </td>
If you are using super orders (a must if you have a lot of orders in my view!) then it is admin/super_orders.php line 997 (approx)
replace
echo zen_draw_checkbox_field('notify', '', true); echo ' ' . ENTRY_NOTIFY_CUSTOMER . '<br />';
with
echo zen_draw_checkbox_field('notify', '', false); echo ' ' . ENTRY_NOTIFY_CUSTOMER . '<br />';
Hope this helps