I guess we will have to wait for someone using vista and IE7, but again avast ‘shouldn’t’ have any effect on format, it doesn’t care, it doesn’t check, it just monitors http traffic the browser handles page format.

Have you tried with firefox or opera under vista ?
Is the site in anyway optimised for IE or use activeX, etc. as MS made some changes to how IE7 handles activeX, this has in some cases broken some sites.