In your graphic card controle center, you can set the quality that the graphics are rendered at to a degree.
For example you can lower the quality of the textures on 3D objects for a preformance increase.
However, as many games uses graphics as the core for their engine, one can not really lower it that much below the already lowest possiable graphics unless (in the case of TES oblivion) you can mod lower graphics into the game.