If you have access to IIS (Internet Information Services) and you can see the current website settings, that  is half the battle.   However, I do not know your IIS environment, so this is just one approach, but easiest in my opinion.  (Note that I use windows server, and I am not familiar with using IIS on XP)

In IIS you can add host headers settings for each domain used to access the installation (on the existing website, right click properties, then under the properties Website Tab, use the button for Advanced).   Set the IP address, port 80, and the host header (domain name) for all addresses possible for any portal for the install, with and without the www if you like.  Then, go into DNN under the host account, to the portals tab and create a new "Parent" portal, setting the alias to equal what was input in the host header field.    This also allows you to have multiple portals using the same domain name, such as www,mysite.com and test.mysite.com and another as store.mysite.com, etc.

I always create "parent" portals instead of a "child" portal.   I think the difference is  that a child uses a virtual directory under the website in IIS (not completely sure that is the only difference).

Hope that helps.