In Windows 8, Microsoft changed the UAC slider’s lowest setting from “Disable UAC” to “Hide UAC”.

So, even with the lowest setting programs will still not run with Administrator privileges (like in Windows 7).

Windows’ "Run" dialog with UAC still active.

To disable UAC, execute this PowerShell script as Administrator (e.g. via powershell from an Admin Command Prompt):

Set-ItemProperty -Path "HKLM:\Software\Microsoft\Windows\CurrentVersion\Policies\System" -Name "EnableLUA" -Value "0"

After that restart and UAC is disabled.

Windows’ "Run" dialog with UAC disabled.

Notes:

• Only do this, if you’re aware of the consequences. Disabling UAC may make the system less secure.
• The Windows 8 Store can’t be used anymore if UAC is disabled. (You can, especially, no longer installed Windows 8.1.)

• To reenable UAC, use -Value "1" in the command above.