When hosting a MVC application under IIS 5.1 there are a couple of small changes to make in order for your application to run. Basically IIS 5.1 doesn’t know what to do with the extensionless URLs that are so common with MVC routing so we have to tell IIS that if one is encountered then to throw it at the Framework 4 aspnet_isapi.dll
To accomplish this first ensure that you have installed .NET Framework verison 4
Next, invoke the IIS Manager under Administrative Tools and Right Click on the Virtual directory for your application and hit Properties.
On this screen Click the Configuration Button and you will be presented with the following popup window
The Executable should point to the aspnet_isapi.dll for .NET Framework 4. This is normally found in C:\Windows\Microsoft.NET\Framework\v4.0.30319
Set the Extension to be .* and uncheck the ‘Check that file exists’ checkbox
If the Ok button is still disabled clikc in the Executable text box and the OK button should enable itself. Little Microsoft Bug me thinks!!
Right Save your settings are restart IIS.
Your MVC Application should now run under IIS 5.1