nrcmsg = "Please use the left mouse button to make your selection(s)";
function NRC(b) {
   if(((navigator.appName=="Microsoft Internet Explorer")&&(event.button > 1))
   ||((navigator.appName=="Netscape")&&(b.which > 1))){
   alert(nrcmsg);
   return false;
   }
}
document.onmousedown = NRC;