Ciao e grazie per la risposta!

Quindi devo fare una cosa così?:

Codice PHP:
<head>
....
<script type="Javascript">
if(document.getElementById("nome").value != "") alert("errore");
if(document.getElementById("cognome").value != "") alert("errore");
if(document.getElementById("pass").value != "") alert("errore");
if(document.getElementById("mail").value != "") alert("errore");
</script>
....
</head>
<body>
<table width="300px"> 
<tr><td width="50%">Nome:</td><td width="50%"><input type="text" id="nome" /></td></tr> 
<tr><td width="50%">Cognome:</td><td width="50%"><input type="text" id="cognome" /></td></tr> 
<tr><td width="50%">Password:</td><td width="50%"><input type="password" id="pass" /></td></tr> 
<tr><td width="50%">Email:</td><td width="50%"><input type="text" id="mail" /></td></tr> 
</table>
</body>