PHP Internet Explorer Compatibility Problem?

[fuggles]

<?php
	session_start();
	$password = "password";
	
	if($_POST['password']==$password) {
		$_SESSION['loggedin']=1;
		header("location:index.php");
	}
	else {
		header("location:index.php");
	}
?>

That works fine in other browsers. But in IE, when you click the submit button the previous page it redirects you back, and not logged in. Internet explorer says something about security. Does anyone know anything about this?

[is_set]

what
try cookies

[fuggles]

It works with all Internet Explorer versions except on Windows 7. Is anyone aware of any problems with Win7 and the "header" function?