I think that you are looking for something like this:
function rightclick() {
var rightclick;
var e = window.event;
if (e.which) rightclick = (e.which == 3);
else if (e.button) rightclick = (e.button == 2);
alert(rightclick); // true or false, you can trap right click here by if comparison
}
(http://www.quirksmode.org/js/events_properties.html)
And then use the onmousedown even with the function rightclick() (if you want to use it globally on whole page you can do this <body onmousedown=rightclick(); >