I want my AltGr key to behave exactly like left Alt.
Usually, I do this kind of stuff with Autohotkey, but I\'m open to different solutions.
As pointed out by PhiLho, Windows provides a way to remap any key, through the registry key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout. A basic overview can be found at Scan Code Mapper for Windows. A better description is probably Answers to Scancode Mapping or Changing Key Values.
I'm using this approach to put the Windows Key on the Caps Lock, because my keyboard doesn't have a Windows Key and I don't need the Caps Lock.