loade hele siden??

lav dine links som:

<a href='index.php?iframe=login'>login</a>

og så din iframe som:

$iframe =  $_GET['iframe'];

if ($iframe != NULL){
$iframe_file    = $iframe.'.php';
    if (file_exists($iframe_file)) {
        $include_iframe_file = $iframe_file;
    } else {
        $include_iframe_file = 'fejl.php';
    }
}
else {
$include_iframe_file = main.php;
}

<iframe src='$include_iframe_file'></iframe>

Eller måske bare:

<form action="" method="" target="_top">