Basically, and according to this interview of Josh Bancroft with Intel’s Chuck DeSylva, the 915 does not meet Microsoft Requirements to run Aero.
I feel that I should remind you, my dear reader, that Windows Movie Maker version 2.0 was running on XP in our devices and now, suddenly, with Vista our same devices do not have enough resources.
I really do not like when somebody takes decisions for me. It makes me feel stupid. Is not easier to give users the power to decide what's acceptable performance?