Thought No.2: I can't see anything obvious in your screen pics so try setting your render settings a bit higher, so anti alias is good, raytrace is full quality etc etc - just pump it all up. Then render at a much higher output - it makes quite a difference for single images - I normally do a test render at around 600px to get the idea, then a big render at around 2000px if I like the small one.

If that fails to improve things, make sure you haven't got anything in the scene settings turned on that shouldn't be, such as lighting effects or lens effects, and do the same check with the shaders, to make sure you're not looking at a lot of graininess that is actually in the objects shaders themselves, rather than the render. I think that makes sense???