I did some further testing, I also cannot load the Smatermail demo site, http://maildemo.smartertools.com/, gives me the same problem that the page does not finish loading (IE7, firefox 2, and Opera 9 under Vista). Testing the url on a XP pc everything works fine.
Testing the Smartermail url from Crystaltech, http://mail.crystaltech.com/, works fine.
Maybe a firewall issue ?