function obtener_ancho()
{
if (navigator.userAgent.indexOf("MSIE") > 0)
{
return(document.body.clientWidth);
}
else
{
return window.outerWidth;
}
}
function obtener_alto()
{
if (navigator.userAgent.indexOf("MSIE") > 0)
{
return(document.body.clientHeight);
} 
else
{
return(window.outerHeight);
}
}
var popup = new Object()
function CenterPopup(URL, width, height)
{
var X = obtener_ancho() / 2
var Y = obtener_alto() / 2
popup = window.open(URL, 'PopUp', 
'scrollbars=yes ' +
'width=' + width + ' ' +
'height=' + height + ' ' +
'top=' + (window.screenTop + (Y - (height/2))) + ' ' +
'left=' + (window.screenLeft + (X - (width/2))) 
)
popup.focus()
}