Quite often when you create a secure page all you have to do is change http to https and you see your URL in green in firefox. Sometimes it happens that after you put https your URL turns red instead of green and in IE when viewing this page you get this annoying popup saying that this page contains both secure and nonsecure items.

There is quite a few things you need to do to make sure your site has only secure items:

1. Change all http references to https, for google analytics use https://ssl.google-analytics.com/urchin.js and not nonsecure http://www.google-analytics.com/urchin.js, all absolute paths to images JavaScript files, CSS files…
2. Get FireBug for firefox to make sure your page does not make any AJAX http requests, and if so change them to https.
3. Look for any broken links or missing resources and fix them.

The problem lies in any unsecure reference which triggers the alert.