HTTP Error 500.21 - Internal Server Error Handler "PageHandlerFactory-Integrated" has a bad module "ManagedPipelineHandler" in its module list. 

Solution:

Run "aspnet_regiis.exe" program from command line (within Command Prompt):

run %windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_regiis.exe –i

 Note: if your computer is 64 bit, then use the following:

%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe –i

Restart IIS.

If above solution does not work for you try the other method below:

Click Start -> All Programs -> Accessories -> Run
In the Open textbox paste in the following line

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

Click OK to start the repair
After, the repair ran for a few minutes, restart IIS.