Your issue is that IIS 5/6 don't play nice with routes without extensions, the home page is resolving because its pointing to default.aspx,
In a nutshell, do this:
If *.mvc extension is not registered to the hosting , it will give 404 exception. The working way of hosting MVC apps in that case is to modify global.asax routing caluse in the following way.
routes.Add(new Route("{controller}.mvc.aspx/{action}",
new MvcRouteHandler())
{ Defaults = new RouteValueDictionary (new{ controller = "YourController"} ) });
In this way all your controller request will end up in *.mvc.aspx, which is recognized by your hosting. And as the MVC dlls are copied into your local bin , no special setttings need to be done for it.
See this question for lots of good information:
http://stackoverflow.com/questions/57712/mvc-net-and-iis-5