Antonio,
This might have something to do with Inno setup.
I used this function when I created the installer:
function GetIISVersion(): String;
var
sVersion: String;
begin
sVersion := '';
GetVersionNumbersString( ExpandConstant('{sys}\inetsrv\inetinfo.exe') , sVersion );
Result := sVersion;
end;
to detect the version number of inetinfo.exe.
I'm not sure (haven't got a 64-bit operating system either), but I think it is necessary for 64-bit systems to use the {sys64} constant to check in the proper directory.
So there would have to be a check if the operating system is 64-bit and act accordingly.