Try This:
Put This In The Body:
<BODY onload="document.body.focus();" onkeydown="AltDown();">
This Is The Function To Detect The ALT:
function AltDown() {
if (window.event.altLeft) {
alert("altLeft Pressed");
}
else {
if (window.event.altKey) {
alert("altRight Pressed");
}
}
window.event.returnValue = false;
document.body.focus();
}