En lugar de onclick deberás usar onmousedown. Un ejemplo:
Código PHP:
<!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=iso-8859-1" />
<title></title>
<script>
document.oncontextmenu=function(){return false;}
window.onload=function(){
document.getElementById('pp').onmousedown=function(event){
e=event || window.event;
(e.button<2)?alert('izquierdo'):alert('derecho');
}
}
</script>
</head>
<body>
<div id="pp">hola</div>
</body>
</html>