Prova così:

Codice PHP:
<?php
    error_reporting(E_ERROR);
    $connect=mysql_connect("localhost","root","");
    mysql_select_db("forum",$connect);
    $query="SELECT* FROM utenti WHERE Username='".$_POST['Username']."'";
    $ris=mysql_query($query);
    if(mysql_num_rows($ris)>0){
        echo "Username gia in uso";
        }
        else{
            if(strcmp($_POST['Password'],$_POST['Conferma_Password'])!=0){
            echo "Le due Password Coincidono";
            }
            else{
            }
        }

?>