Hey, perhaps someone can help me with this!

I'm trying to set up a SharePoint 2007 application with the following url:
http://mysub.mysite.com:81," target="_blank" rel="nofollow">http://mysub.mysite.com:81,

but I just don't seem to get it to work. When I try to add a new application and enter "http://mysub.mysite.com:81" as host-header, add a site collection, and then try to browse the site, I am asked for credentials, I enter them, am asked for them again, repeat three times, and then a blank page comes up (401 unauthorized access). I am setting this up on a virtual machine and I don't need for this to be accessed from outside.

I have a Windows 2008 R2 Server
SQL Server 2008
SharePoint 2007