jQuery would be your best bet for something like this, for example if you had a page with div id "main" in which you wanted to display your content and a text file called 'main-content.txt' the entire code to display this (after including jQuery on your page) would be:
// *the $(document).ready(function() { part is only included once per page*
$(document).ready(function() {
$('#main').load('/path/to/main-content.txt');
});