Grazie, ma non mi funziona. Sicuramente sbaglio io qualcosa, sei gentile, potresti controllare queste mie instruzioni (inserite nel file utenti.html) ?

<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Accesso</title>
</head>
<body>
&nbsp;
<br>&nbsp;
<br>&nbsp;
<center>


<script>
function Log(){
var user = document.getElementById("user").value;
var psw = document.getElementById("psw").value;
if(user!="" && psw!=""){
document.modulo.action=http://www.miositodiprova.it/user+psw;
document.modulo.submit();
}else{alert("compila user e password"); return false;}
}
</script>
<form method="get" action = "" name="modulo" onsubmit="Log()"> campi form psw e user tasto tipo submit</form>


<b><font face="Arial,Helvetica"><font size=+1>Username:</font></font></b>
<br><input type="text" name="user" size="30"/>
<br>&nbsp;
<p><b><font face="Arial,Helvetica"><font size=+1>Password:</font></font></b>
<br><input type="password" name="psw size="30"/>
<p><input type="submit" value="Invia" /></form>
<br>&nbsp;



</body>
</html>