prompt in IE8 for secure pages produces altered layout
The message you're getting in IE 8 is due to the fact that you're viewing your site through HTTPS, but some of your site's assets / resources are still being loaded through HTTP. If you look at the site in Chrome, you'll notice an icon to the right of the address bar which indicates the same warning about insecure content.

I would recommend going through all your templates / custom modules to make sure that any JS, CSS and iFrames (hope you have none of those heh) are all being served via HTTPs and not HTTP. Once nothing is being served via regular HTTP you shouldn't get that message anymore.

Hope this helps!