Hello Guys i tried to implement in comment box but i failed i got this code from Russian site and it works in only in registration page ..so if any body can implement in comment section it will be great and please share it here if you did.. thanks.
http://www.google.com/recaptcha
Login to recaptcha using google account add your domain after download latest PHP library version at
http://code.google.com/p/recaptcha/downloads/list?q=label:phplib-Latest
Extract zip and upload recaptchalib.php to
root/example/engine/modules/
Installation:
Open
engine/modules/register.php
Find
require_once ENGINE_DIR . '/classes/parse.class.php';
BELOW add
require_once ENGINE_DIR . '/modules/recaptchalib.php';
Find
function check_reg($name, $email, $password1, $password2, $sec_code = 1, $sec_code_session = 1) {
Replace with
function check_reg($name, $email, $password1, $password2,$sec_code=1) {
Find
if( $sec_code != $sec_code_session or ! $sec_code_session ) $stop .= $lang['reg_err_19'];
Replace with
if( $sec_code != 1) $stop .= $lang['reg_err_19'];
Find
if( $config['allow_sec_code'] == "yes" ) {
        $sec_code = $_POST['sec_code'];
        $sec_code_session = ($_SESSION['sec_code_session'] != '') ? $_SESSION['sec_code_session'] : false;
    } else {
        $sec_code = 1;
        $sec_code_session = 1;
    }
        $sec_code = $_POST['sec_code'];
        $sec_code_session = ($_SESSION['sec_code_session'] != '') ? $_SESSION['sec_code_session'] : false;
    } else {
        $sec_code = 1;
        $sec_code_session = 1;
    }
Replace (Add your Private Key )
if( $config['allow_sec_code'] == "yes" ) {
          $resp = recaptcha_check_answer ("Private Key",$_SERVER["REMOTE_ADDR"],$_POST["recaptcha_challenge_field"],$_POST["recaptcha_response_field"]);
          if ($resp->is_valid)
            $sec_code=1;
          else
            $sec_code=2;
    }
          $resp = recaptcha_check_answer ("Private Key",$_SERVER["REMOTE_ADDR"],$_POST["recaptcha_challenge_field"],$_POST["recaptcha_response_field"]);
          if ($resp->is_valid)
            $sec_code=1;
          else
            $sec_code=2;
    }
Private Key add by you, don't forget to do this
Find
$tpl->set( '{reg_code}', "<span id="dle-captcha"><img src="" . $path['path'] . "engine/modules/antibot.php" alt="{$lang['sec_image']}" border="0" /><br /><a onclick="reload(); return false;" href="#">{$lang['reload_code']}</a></span>" );
Replace: (Add your Public Key )
$tpl->set( '{reg_code}', recaptcha_get_html("Public Key", "") );
Public Key add by you, don't forget to do this
Find and remove below code
function reload () {
    var rndval = new Date().getTime();
    document.getElementById('dle-captcha').innerHTML = '<img src="{$path['path']}engine/modules/antibot.php?rndval=' + rndval + '" border="0" width="120" height="50" alt="" /><br /><a onclick="reload(); return false;" href="#">{$lang['reload_code']}</a>';
};
    var rndval = new Date().getTime();
    document.getElementById('dle-captcha').innerHTML = '<img src="{$path['path']}engine/modules/antibot.php?rndval=' + rndval + '" border="0" width="120" height="50" alt="" /><br /><a onclick="reload(); return false;" href="#">{$lang['reload_code']}</a>';
};
Now final step.. Open your template folder registration.tpl and find this code and remove it...it looks some thing link this
<tr valign="middle">
  <td width="120" height="25"><br>Enter the code</td>
  <td height="25"><br><input type="text" name="sec_code" style="width:167px; height:18px; font-family:tahoma; font-size:11px; border:1px solid #E0E0E0 "></td>
</tr>
  <td width="120" height="25"><br>Enter the code</td>
  <td height="25"><br><input type="text" name="sec_code" style="width:167px; height:18px; font-family:tahoma; font-size:11px; border:1px solid #E0E0E0 "></td>
</tr>
Thats all ....Thanks once again...
Updates 
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