How to dynamically load content like portal?

I need to develop a web site in ASP.NET which will need to be constructed like a portal in which you have the main static page which then, depending on the requested URL, will load a certain page in the designated area. Is this done with different Include statements in a select case? Or is there some more elegant way to implement this without using a 3rd party system? Also, this can be done using frames, it must end up being a single document in the user's browser.

For example, let's say the portal page is called index.aspx. Then, if someone calls index.aspx?page=home, then the content of home.htm (or home.aspx for that matter), will be inserted into the index.aspx page at a designated place. So when the user finally sees the document, it's a single document

Any help or pointers would be appreciated.

dotnetnuke is a good example to pull ideas from...