Find:
$enquiry = tep_db_prepare_input($HTTP_POST_VARS['enquiry']);

Add after:
$protect = tep_db_prepare_input($HTTP_POST_VARS['protect']);
$lock = tep_db_prepare_input(base64_decode($HTTP_POST_VARS['lock']));

if ($protect != $lock) {
  $error = true;

  $messageStack->add('contact', ENTRY_EMAIL_HUMAN_CHECK_ERROR);
}

Find:
<?php echo tep_draw_form('contact_us', tep_href_link(FILENAME_CONTACT_US, 'action=send'), 'post', '', true); ?>

Add after:
<?php
$a = rand(1, 9);
$b = rand(1, 9);
echo tep_draw_hidden_field('lock', base64_encode(($a + $b)));
?>

Find:
<tr>
  <td class="fieldKey" valign="top"><?php echo ENTRY_ENQUIRY; ?></td>
  <td class="fieldValue"><?php echo tep_draw_textarea_field('enquiry', 'soft', 50, 15); ?></td>
</tr>

Add after:
<tr>
  <td class="fieldKey" valign="top"><?php echo sprintf(ENTRY_HUMAN, $a, $b); ?></td>
  <td class="fieldValue"><?php echo tep_draw_input_field('protect'); ?></td>
</tr>