Well what I did was I originally installed DNN as a virtual directory and then decided that I wanted to run it directly under the default directory. If you're using IIS the default directory is typically c:\inetpub\wwwroot.  Make sure you assign that folder the appropriate permissions, it depends on what you're running. If you're running an XP box then I believe you have to grant the ASP.NET account modify permissions. If it's Windows 2003 server the grant the Network Service account modify permissions. You might want to double check the required permissions since it's been awhile since I installed mine.

Another couple of snags I had was within IIS itself. Open IIS, expand the tree until you see default website, right click and go into properties. Under the Home Directory tab make sure you have "script source access" checked. Next go into the Documents tab. Make sure "default.aspx" is on that list and is first in line. If not make sure you add it.

Ok now you should have access to your database. Open the DB and look for the "PortalAlias" table. There should be three columns the last one is the most important, HTTPAlias.  Make sure you have an entry for your domain (i.e. mydomain.com). If you're using a virtual directory you'll most likely have to have a second alias in there (i.e. mydomain.com/dnn). If you're using IP's you can probably access your portal by punching in that IP and then going in to the host settings and changing that HTTP alias.

I hope this helps. Reread some of the previous posts in this thread because those guys know more about this then I do. Good luck!