A massive amount of post out there describes how easy it is to change from the default (cmd.exe) to Powershell when using the “Power User Menu” (Win+X) in Windows 8.1 and Server 2012 R2. One example you find here from my friend Daniel.
I honestly don’t know why they didn’t set PowerShell as the default Shell, which actually was default in the Preview.

Rolling out Windows 8.1 (or Server 2012 R2) in an Enterprise, you would of course want this as the default setting for all users.
Doing the change in the UI, the registry key
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced\DontUsePowerShellOnWinX is set to 0.

Here is how to do it with group policy preferences
* The GP Preferences (except the PowerShell-line) can be created from any machine with gpmc newer than Server 2008. The other policy requires you to have the latest .admx-files or do the change from a Server 2012 R2 Machine

In Group Policy Management, in a new or in an already existing user policy, navigate to User Congfiguration\Preferences\Windows Settings\Registry and create a new registry item configured as below.
(the entire key path used is “Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced“)

If you want to do the same thing as above against an existing policy, you can do it with the following PowerShell line:

Set-GPPrefRegistryValue -Name "YourPolicy" -Context User -Key HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Advanced -ValueName DontUsePowerShellOnWinX  -Value 0 -Type DWORD -Action Update

If you instead want to prevent your users from changing the shell, there is a policy setting for that under
User Configuration\Administrative Templates\Windows Components\Edge UI 

Why you would want prevent your users from changing from cmd to PowerShell I don’t know, but someone obviously wanted cmd.exe as the default one…
Note that the registry value for the PowerShell menu does not work if you enable this policy!