HTTP Error 500.21 - Internal Server ErrorHandler "PageHandlerFactory-Integrated" has a badmodule "ManagedPipelineHandler" in its module list

This is the error due to not installing the .net frame work 4.0 on IIS7.x server. To install the .net 4.0 frame work run the following on command prompt.
on 32-bit operating system
%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe –i


on 64-bit operating system
%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe –i

If the above solution not worked then apparently, the reason for recieving the Internal Server error message was that installed SQL Server 2008, after installing Visual Studio 2010 vice versa, and because of this it corrupted the IIS Machine level configuration files ("If you install VS2010 and then install VS2008 and VS2008 SP1, the configuration files for ASP.NET in IIS only include about 1/2 of the correct .Net 4.0 configuration sections." read more here).

To repair this problem run a full silent repair of the .NET Framework 4.0.   Here's how on either a 32 bit or 64 bit computer:

1. Click Start -> All Programs -> Accessories -> Run
2. In the Open textbox paste in the following line (see list of all .NET Framework version install, repair and unistall command lines here):
   For silent repair on 32 bit computer with .Net Framework version 4.0.30319 use:
   %windir%\Microsoft.NET\Framework\v4.0.30319\SetupCache\Client\setup.exe /repair /x86 /x64 /ia64 /parameterfolder Client /q /norestart
   
   For silent repair on 64 bit computer with .Net Framework version 4.0.30319 use:
   %windir%\Microsoft.NET\Framework64\v4.0.30319\SetupCache\Client\setup.exe /repair /x86 /x64 /ia64 /parameterfolder Client /q /norestart
   
   
3. Click OK to start the repair
4. After, the repair ran for a few minutes, I restarted IIS 7.5, and things began to work correctly!

Hopefully, this will work for you...