Custom Alert box with javascript

Hi,
 
I’m using a Custom Alert code, from this website, and I need a bit of help.
Custom Alert
 
I have a simple script that allow redirect users to different page if browser is not the correct to work property with my site.
 
The code works perfect, but I can’t stay in the same page after load the Alert Box.
I need to this custom Alert stay in same page, and after clic on button “Ok” the user should be redirect depending of the browsers. But this not happening, the alertbox is showed and later hide.
 
Any clue?
 
This is the code to redirect users to different pages depending of browser.

var bname = navigator.appName;
var bcode = navigator.appCodeName;
var bplat = navigator.platform;
var bagen = navigator.userAgent;
 
if (bname == "Microsoft Internet Explorer")
   {
alert("Estas usando " + bname + "\nPor favor usa el navegador Mozilla Firefox" + "\nExiste problemas de incompatibilidad" + "\nGracias por tu comprensión");
   window.location="page1.html";
   }
else
   {
if (bagen== "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US) AppleWebKit/534.13 (KHTML, like Gecko) Chrome/9.0.597.98 Safari/534.13")
   {
alert("Estas usando Google Chrome"+ "\nPor favor usa el navegador Mozilla Firefox" + "\nExiste problemas de incompatibilidad" + "\nGracias por tu comprensión");
   window.location="page2.html";
 
   }
else
   {
if (bcode == "Mozilla")
   {
   }
else
{
alert("Estas usando un navegador no apropiado para este sistema" + bname + "\nPor favor usa el navegador Mozilla Firefox" + "\nExiste problemas de incompatibilidad" + "\nGracias por tu comprensión");
   window.location="page3.html";
   }
   }
}

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And this is the code from the author of Alert Box, this use some css styles to design the alert box, But in this case this is unnecessary. I just paste the java code.

// constants to define the title of the alert and button text.
var ALERT_TITLE = "Oops!";
var ALERT_BUTTON_TEXT = "Ok";

// over-ride the alert method only if this a newer browser.
// Older browser will see standard alerts
if(document.getElementById) {
	window.alert = function(txt) {
		createCustomAlert(txt);
	}
}

function createCustomAlert(txt) {
	// shortcut reference to the document object
	d = document;

	// if the modalContainer object already exists in the DOM, bail out.
	if(d.getElementById("modalContainer")) return;

	// create the modalContainer div as a child of the BODY element
	mObj = d.getElementsByTagName("body")[0].appendChild(d.createElement("div"));
	mObj.id = "modalContainer";
	 // make sure its as tall as it needs to be to overlay all the content on the page
	mObj.style.height = document.documentElement.scrollHeight + "px";

	// create the DIV that will be the alert 
	alertObj = mObj.appendChild(d.createElement("div"));
	alertObj.id = "alertBox";
	// MSIE doesnt treat position:fixed correctly, so this compensates for positioning the alert
	if(d.all && !window.opera) alertObj.style.top = document.documentElement.scrollTop + "px";
	// center the alert box
	alertObj.style.left = (d.documentElement.scrollWidth - alertObj.offsetWidth)/2 + "px";

	// create an H1 element as the title bar
	h1 = alertObj.appendChild(d.createElement("h1"));
	h1.appendChild(d.createTextNode(ALERT_TITLE));

	// create a paragraph element to contain the txt argument
	msg = alertObj.appendChild(d.createElement("p"));
	msg.innerHTML = txt;
	
	// create an anchor element to use as the confirmation button.
	btn = alertObj.appendChild(d.createElement("a"));
	btn.id = "closeBtn";
	btn.appendChild(d.createTextNode(ALERT_BUTTON_TEXT));
	btn.href = "#";
	// set up the onclick event to remove the alert when the anchor is clicked
	btn.onclick = function() { removeCustomAlert();return false; }

	
}

// removes the custom alert from the DOM
function removeCustomAlert() {
	document.getElementsByTagName("body")[0].removeChild(document.getElementById("modalContainer"));
}

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Any help will be welcome.
Thanks.