What is wrong with the following code..it does not work for me?

PHP Code:
<HTML>
<HEAD>
<SCRIPT language="JavaScript">
    function GetKeyCode() { 
        {
        alert("KeyCode is " + document.event.KeyCode); 
}
    document.keydown=GetKeyCode(); 
} 

</SCRIPT>
</HEAD> 
<BODY onload="GetKeyCode()"> 
<P> Press the key you want to know the KeyCode 
</BODY>
</HTML>