Check out this: 

Go to IIS ->properties on your web site -> Home Directory -> Configuration...

 Press the Edit... button under "Wildcard application maps" and make sure that the "Verify that file exists" is NOT checked for your aspnet_isapi.dll file.

--
Lars Øyvind 

Hi Lars Øyvind!

Thanks for the reply!

The problem is that the error occurs during the install so I haven't even got a web site installed on my IIS. I thought the creation of a web site in IIS was supposed to be taken care of by the EPiServer installer?

/Thomas Kahn

If you install IIS after .net mapping will not be done properly.

1. go to cmd

1. go to C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727(  if you have installed 2.0, otherwise go to the particular version folder)

3. Type aspnet_regiis.exe

It will start installing the regiis and maaping will be taken care by itself.

Hope this will be useful

Thanks
Lakshmi Narayanan

Hi,

 We got this error here for the heinous crime of having the pre-SP1 version of the .NET Framework installed.

If, in %WINDIR%\Microsoft.NET\Framework\ you have two directories: v2.0.50215  and v2.0.50727 then the R2 installer will bomb out with this error.

To stop it, simply temporarily move the v2.0.50215 directory outside of the Framework directory (dump in on the desktop or something), run the installer, and then put it back before anyone else relying on it realized it was your fault their app stopped working :)

 Hope this helps!

Andy