ecco il codice corretto:

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<HTML LANG="IT">
<HEAD>
  <TITLE> </TITLE>
  <META http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">

<script>
function abilitaDisabilita(valore, f) {
  f.pippo.disabled=!valore;
  f.pluto.disabled=!valore;
  f.paperino.disabled=!valore;
  f.gambadilegno.disabled=!valore;
}
</script>

</HEAD>
<BODY>
<form>
<input type="text" name="pippo" disabled>
<input type="text" name="pluto" disabled>
<input type="text" name="paperino" disabled>
<input type="text" name="gambadilegno" disabled>
<input type="checkbox" onclick="abilitaDisabilita(this.checked, this.form);">
</form>
</BODY>
</HTML>
ciao