From：http://support.microsoft.com/kb/2015129
Symptoms
Consider the following scenario. You install the .NET Framework 4.0. Then, you install an earlier version of the .NET Framework, or you enable .NET 3.0 WCF HTTP Activation. In this scenario, you may receive the following error message when you when run applications that are hosted on Internet Information Services (IIS):
Could not load type 'System.ServiceModel.Activation.HttpModule' from assembly 'System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.Exception Details: System.TypeLoadException: Could not load type 'System.ServiceModel.Activation.HttpModule' from assembly 'System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.
Cause
This issue occurs because the Applicationhost.config file for Windows Process Activation Service (WAS) has the following section defined, and this section is incompatible with the .NET Framework 4.0:
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />
This section should be defined as follows:
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler,runtimeVersionv2.0" />
Note You can find the Applicationhost.config file in the following location:
%windir%\system32\inetsrv\config
Resolution
To resolve this issue, run the following command line:
aspnet_regiis.exe /iru
The Aspnet_regiis.exe file can be found in one of the following locations:
%windir%\Microsoft.NET\Framework\v4.0.30319
%windir%\Microsoft.NET\Framework64\v4.0.30319 (on a 64-bit computer)
注意
以上方案在win8系統中無效，因為在最后一步運行 aspnet_regiis.exe /iru 命令時會失敗，所以解決方案是找到 Applicationhost.config 文件（目錄在%windir%\system32\inetsrv\config），然后把文件中的：
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler" />
改為：
<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler,runtimeVersionv2.0" />
我目前只找到這種方案，如果有更好方案歡迎指出。