IIS 7.5 : HTTP Error 500.21 - Internal Server Error - Handler "PageHandlerFactory-Integrated" has a bad module "ManagedPipelineHandler" in its module list

Solution:

This is is because ASP.Net was not completely installed with IIS. 

To fix this, simply run the following command at the command prompt

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

If I had been on a 32 bit system, it would have looked like the following:

%windir%\Microsoft.NET\Framework\v4.0.21006\aspnet_regiis.exe -i

Once this has been done, Restart IIS and run the web application.