Ever started cmd.exe to see this: “The command prompt has been disabled by your administrator”?
When started, cmd.exe checks for the existence of a certain key in the registry and decides to continue execution based on the value of this key. This key is DisableCMD and sits in Software\Policies\Microsoft\Windows\System. For regedit.exe, the key is DisableRegistryTools in Software\Microsoft\Windows\CurrentVersion\Policies\System.
There are many hacks to bypass this technique, depending on what kind of control you have as a user. When you have only control over the content of the programs you execute, use this trick: edit a copy of cmd.exe with a binary editor, search for DisableCMD and change it to something else, like DisableAMD. This copy of cmd.exe will now look for a key that doesn’t exist, and thus continue execution. For regedit, I renamed the key to DisableRegistryFools.
Mark Russinovich has another, elegant hack for this: he starts the program and injects a DLL that intercepts calls to the registry API and filters the return values. Limited users can inject DLLs into their own processes. But since Microsoft bought Sysinternals, his tool (GPdisable) is not available anymore.