Not sure if this is what is causing the issue or what order you installed and setup IIS and .NET, but you might want to make sure that you have registered the framework with IIS properly have you installed IIS.

Registering ASP.NET on IIS after installing the .NET Framework

Assuming you are trying to install SSRS 2005. Just an FYI, SSRS 2008 does not require IIS anymore:)