Gabe forwarded me a pointer to a cool utility he worked on which enables you to define and switch between multiple sites with IIS on Windows XP -- which out of the box only allows you to define one site (note: you can have multiple applications underneath the site -- but only one unique port or hostname combination). The utility allows you to define multiple sites, and then quickly switch which site is "active" at a particular time.
You can download the utility as well as the C# source code for it here.
Hope this helps,
P.S. I always thought the one site limitation was silly. We are changing to support multiple sites on Windows client with IIS7...