From：http://support.microsoft.com/kb/2015129

Symptoms

　　Consider the following scenario. Youinstall the .NET Framework 4.0. Then, youinstall an earlier version of the .NET Framework, or you enable.NET 3.0 WCF HTTP Activation. In this scenario, you may receive the followingerror message when you when runapplications 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 .NETFramework4.0:

<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" />

　　NoteYou 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系统中无效，因为在最后一步运行aspnet_regiis.exe /iru 命令时会失败，所以解决方案是找到Applicationhost.config 文件（目录在%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" />

　　改为：

<add name="ServiceModel" type="System.ServiceModel.Activation.HttpModule, System.ServiceModel, Version=3.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" preCondition="managedHandler,runtimeVersionv2.0" />

　　我目前只找到这种方案，如果有更好方案欢迎指出。
作者：Artwl
出处：http://artwl.cnblogs.com