$user = $_POST['user'];
    $pass = md5($_POST['pass']);
    if(empty($user) or empty($pass)) echo "Preencha todos os campos para continuar!";
    else {
    $dbhost = "localhost";
    $dbuser = "user";
    $dbpass = "senha";
    $dbname = "banco";
    $con = mysql_connect($dbhost, $dbuser, $dbpass) or die(mysql_error());
    $db = mysql_select_db($dbname, $con) or die(mysql_error());
    $query = mysql_query("INSERT INTO tbl_users VALUES (NULL,'$user','$pass')");
    if($query) echo "Sua conta foi criada com sucesso!";
    }

<form name="form3" method="post" action="cadastrar.php">
    <label>USER:</label>
    <input type="text"  name="user"/>
    <label>PASS:</label>
    <input type="password" name="pass" />
    <input type="submit" value="CADASTRAR" />
    </form>

    tabela.sql

    CREATE TABLE `tbl_users` (
    `id` int(11) NOT NULL auto_increment,
    `username` varchar(250) collate latin1_general_ci NOT NULL,
    `password` varchar(250) collate latin1_general_ci NOT NULL,
    PRIMARY KEY  (`id`)
    ) ENGINE=MyISAM  DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci AUTO_INCREMENT=6 ;

$query = mysql_query("INSERT INTO tbl_users VALUES ('$user','$pass')");
    if($query) echo "Sua conta foi criada com sucesso!";
    else echo "Usuário já existe, escolha outro nome.";

CREATE TABLE `tbl_users` (
`id` int(11) NOT NULL auto_increment,
`username` varchar(250) collate latin1_general_ci NOT NULL UNIQUE KEY,
`password` varchar(250) collate latin1_general_ci NOT NULL,
PRIMARY KEY  (`id`)
) ENGINE=MyISAM  DEFAULT CHARSET=latin1 COLLATE=latin1_general_ci AUTO_INCREMENT=6;

ADD UNIQUE INDEX `username` (`username`);