C# ASP.NET : How to load another URL content into a DIV with "AJAX"?

Turrican, you might be coming at this from the wrong side, while you say you want to avoid page loads thats file, but when you talk about includes to avoid having repeating information.. this just screamed out to me that you maybe aren't aware of:

Master pages

Thats only happening because your controls are now sat in a parent control, that would well happen without a master page if you are using server side controls then their client side id is their hierical id (their id, their parents id and so on)..

You should be using ClientID to determine the correct client side id on a webforms application, and javascript in external files should not be aware of the pages structure.. you should instead have stubs on the page that initialize your javascript functionality.. so you would have your js that has a method which takes an element or id as an argument, and then from your page code, you have a small script tag that finds the element or tag, and calls into that js method..

For example, in supercoolwowjavascripteffects.js you might have

function makeElementExplodeWow(element) {
  .. insert code here to make explodes happen on element ..
}

then on your page you would have

<script>
var explodingElement = document.getElementById("<%= txtName.ClientID %>");
makeElementExplodeWow(explodingElement);
</script>