How to fix: Handler "PageHandlerFactory-Integrated" has a bad module "ManagedPipelineHandler" in its module list
It turns out that this is because ASP.Net was not completely installed with IIS even though I checked that box in the "Add Feature" dialog. To fix this, I simply ran 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
Remeber to run the command prompt as administrator (CTRL+SHIFT+ENTER)
The accepted answer is correct, however sometimes you would get the "Aspnet_regiis.exe is not recognized as an internal or external command, operable program or batch file." error message.
To resolve it try the following:
Make sure that your .NET 4.0 installation is not corrupted (run the installer and 'Repair' it). There's also a chance it is not installed on your machine at all.
If you're sure you don't have .NET 4.0 installed and want to run it as .NET 2.0, try this:
If you see the message "Aspnet_regiis.exe is not recognized as an internal or external command, operable program or batch file.", switch to the C:\Windows\Microsoft.NET\Framework64\v2.0.50727\Aspnet_regiis.exe -i at the command prompt.
To solve the issue try to repair the .net framework 4 and then run the command
%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_regiis.exe -i