ho anche aggiustato qualche virgola che mancava, ma nulla..è immobile:

< !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;