Prova cosi.

codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
		<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
		<title>Test tasto premuto</title>
		<script type="text/javascript">
			function pressione(evento)
			{
				var chiave;
				if(!evento)
				{
					evento = window.event;
				}
				if(evento.keyCode)
				{
					chiave=evento.keyCode;
				}
				else
				{
					chiave=evento.charCode;
				}
				alert(chiave);
			}
		</script>
	</head>
	<body>
		<input type="text" onkeypress="pressione(event)">
	</body>
</html>