Inloggnings problem med md5 password

Hej jag har haft problem med md5 passwords. Allt funkar bra vid reggning och koden är krypterad på databasen men när jag ska logga in så failar det, matchar inte lösen?

INLOGGNINGS SIDAN //

<?php

    if ($_POST['username'] && $_POST['password'])
	{  
        mysql_connect("xxxxxx.se.mysql","my_db","pass");
        mysql_select_db("my_db");			

		$username = post_string('username'); 
		$password = post_string('password'); 
		$password = md5($password);			
		
								
        $sql = mysql_query("SELECT * FROM users WHERE username = '$username' AND password = '$password' LIMIT 1");
        
        if (mysql_num_rows($sql)>0)
        {
            echo ("Welcome, you are logged in as $username!");
        }
            else 
            {
                echo "Login failed, please try again.";
            }
    }

?>

REGG SIDAN //

<?php

	function post_string($str) {	//funktion som hämtar och saniterar post
	if(!isset($_POST[$str])) { return; }
		if (!get_magic_quotes_gpc()) {
				$return_str = addslashes($_POST[$str]);
			} else {
				$return_str = $_POST[$str];
		} return $return_str; 
	}
	
	
	    mysql_connect("xxxxx.se.mysql", "my_db", "pass");
	    mysql_select_db("my_db");
		
		$username = post_string('c_user'); 
		$password = post_string('c_pass'); 
	
		if ($username && $password) {
		
				$password = md5($password);
				$sql = "INSERT INTO users (id, username, password) VALUES ('', '$username', '$password')"; 
				mysql_query($sql) or die(mysql_error());
				
				echo "User created sucsessfully!";
				echo "<br /><br />";
				echo ("Your now known  as:" ." ". "$username");
				echo "<br /><br /><br /><br />";
	
				
				
	}
	
?>