en xp tienen que comprobar si el programa netsend esta activado , si es uno de esos xp remodelados es posible que no este instalado para disminuir tamaño

<?
if(!isset($_POST['submit'])){
  show_form();
}else{
  //get message
  if (isset($_POST['message'])){
     $mess = escapeshellcmd($_POST['message']);
  }else{
     echo "no message";
     show_form();
     die();
  }
  //$ip is the ip of your friends machine...you could build a db table or array to use the name entry to find the ip
  if (isset($_POST['ip'])){
     $ip = escapeshellcmd($_POST['ip']);
  }else{
     echo "no ip";
     show_form();
     die();
  }
  echo "NET SEND $ip $mess";
  exec("NET SEND $ip $mess");

}
function show_form()
{
?>
    <form method="POST" action="<?=$_SERVER['PHP_SELF'];?>">
    <table cellpading=2 cellspacing=2 border=2 width=50%>
    <tr>
    <td align=center valign=top>
    <p>IP<br>
    <input type="text" name="ip" size="20"></p>
    </td>
    </tr>
    </table>
    <table cellpading=2 cellspacing=2 border=2 width=50%>
    <tr>
    <td align=center valign=top>
    <p>Message<br>
    <textarea cols=40 rows=4 name="message"></textarea>
    <p><input type="submit" name="submit" value="Send"></P>
    </td>
    </tr>
    </table>
<?
}
?>