I believe the entire issue is that your Windows 7 machines are cached. Did you try your site when the site wasn't working previously on the Windows 7 machines and not on the Windows XP ones? If so, then you are simply cached to the prior non-working location. Here is how to see if you are cached, go on the Windows 7 machines to a proxy:
http://proxify.com/If you an see your website on the proxy on Windows 7, then you are cached. You can try clearing your computer's DNS cache to see if that helps or reboot the system and router (if you have a router). To clear your cache in Windows 7:
http://www.addictivetips.com/windows-tips/clear-windows-7-cache/Thanks.