function redireccionar() {
var url640x480 = "stv.php";
var url800x600 = "stv.php";
var url960x600 = "stv.php";
var url1024x768 = "st.php";
if ((screen.width == 640) && (screen.height == 480)) 
window.location.href= url640x480;
else if ((screen.width == 800) && (screen.height == 600))
window.location.href= url800x600;
else if ((screen.width == 960) && (screen.height == 600))
window.location.href= url960x600;
else if ((screen.width == 1024) && (screen.height == 768))
window.location.href= url1024x768;
else window.location.href= url1024x768;
}