I am thinking this is either an NTFS permissions problem or it is one with your IIS.  I assume the domain is pointed to the server already and if pinged returns the proper IP.  Second, I would assume you have your local IIS setup to take in the proper host header (domain) or you are taking in all to a single server (default server).  The next problem would be not having Default.aspx setup as the first document in your list (default page).  Let me know once you complete this checklist.

This goes back to IIS again.  If you can access this via localhost/DotNetNuke I am guessing you only have the default virtual server setup meaning all requests first go to your wwwroot and not your dotnetnuke folder.  For www.domain.com/DotNetNuke to work it would require it to be listed as an alias but I still don't think this is the problem here.  Without me actually being able to see how exactly you have everything setup, the best I can suggest is you put a simple default.htm in the wwwroot with some text and one in the dotnetnuke folder with different text and see what happens when you call it up in a web browser directly and hopefully you can get some answers by calling default.aspx as well.