function izquierda(e) {
if (navigator.appName == 'Netscape' && (e.which == 1 || e.which == 2)){
alert('')
return false;
}
else if (navigator.appName == 'Microsoft Internet Explorer' && (event.button == 1)){
alert('')
}
}

if (navigator.appName == 'Mozilla' && (e.which == 1 || e.which == 2)){
alert('')
return false;
}


document.onmousedown=izquierda