Have you noticed substantial differences in website rendering on IE 6 or IE 7 when run on Windows XP versus when run on Windows Vista?
The rendering is the same as far as I've noticed, as long as you compare apples to apples (meaning: IE7 looks the same on both, but IE6 may not look the same as IE7). Vista might do some extra security enforcement, but I'm not aware of any specifics. And there's no such thing as IE6 on Vista.