I second Ikegami. An easy way to do AJAX is using the Prototype JavaScript library, check the Prototype introduction, under the section "Updating your page dynamically with Ajax.Updater".
This example makes a post request to "myscript.pl", sending the content of the form with id "myForm" as parameters. The HTML output of the script will be inserted into the div with id "MyDiv". It's not so difficult :)

function upadateMyDiv() {

    new Ajax.Updater('myDiv', '/myscript.pl', {
        parameters : $('myForm').serialize(true),
        method : 'post',
    });
}
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