What ESRI products are installed on the server?
Run PatchFinder.exe from the ESRI installation media, save the output to a text file, and then post the text file.
Also, please confirm that you have authorized/licensed ArcGIS Server on the web server machine.
Please confirm that you have a 64 bit web application, i.e. you have not enabled 32 bit mode on the application pool which is associated with the web application.
One suggestion is to write a trivial .NET console application which simply makes the call below and have it catch and log any exceptions. Get that to work first and then worry about IIS, ASP.NET, etc.