Does the username and password get carried over in the URL and if it does did you apply the most recent Internet Explorer patch from Microsoft.
If the answer to both is yes then your problem is that the latest patch from Microsoft removes Internet Explorers ability to carry username and password information in any URL. I had the same problem with a website control panel I use.
See
http://www.microsoft.com/technet/sec.../MS04-004.mspx and
http://support.microsoft.com/default...d=kb;EN;834489 for more information.
Hope this helps.