I have accidentally pressed some keyboard shortcut which caused the Alt Gr key to be replaced by plain Alt key behavior. However, I don't know which shortcut it was, even after trying a lot of different combinations. After restarting my PC, the problem is solved, but I fell into this situation a few times now, and it is really inconvenient.
How do I configure the behavior of the right alt key?
Note: I'm working on a Windows Vista machine.