Prova questo

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<script type="text/javascript"> 
window.onload=function(){ 
document.form2.onkeypress = prova; 
} 

function prova() { 
 var tasto = window.event.keyCode; 
 if (tasto == 13) { 
 document.form1.T1.value = "enter"; 
 return false;
 } 
 return true;
} 
</script> 
<FORM name="form1" METHOD="POST" style="padding: 0"> 


 
<input type="text" name="T1" size="20"> form 1</p> 
</form> 
<FORM name="form2" METHOD="POST" style="padding: 0">

 
<input type="text" name="T1" size="20">form 2</p> 
</form> 

</body>
</html>