OK - this seems to be a problem that many people have run into. But for IIS 6 it seems to be some additional configuration that is needed in order to get ASP.NET MVC up and running.
First you’ll need to find the version of IIS. Yes, I know, it should be simple, but it’s not… Here is how you do it.
Then, when you know which one, and if it’s IIS6 or lower (i presume…) you can follow along nicely in this article by Gopinath - who saved my butt.