The photos are grainy most likely due to the dim lighting conditions. The grain(noise) is due to the increased ISO setting of the camera to compensate for the dim lighting conditions so that you can get a sharper(but more noisy) shot in that kind of lighting.
What I would suggest is to turn on the flash(if the subject is not too far away), NOT zoom in when in dimly lit areas(try moving nearer instead) and hopefully this will help.
I'm quite sure that you are still using the Auto mode of your F480, which does not allow you to manually select an ISO setting. When you switch it to SP(scene position), there is one scene position that shows a camera with an 'M' which stands for 'manual'. In this mode, you can select a lower ISO setting of 100 which would allow pictures with less noise. But this may also result in blurrer pictures if you move slightly, or if your subject moves. Anyway, you can try experimenting with the M mode if you haven't, but if you're not too sure, just leave it back in Auto.
The noise that you see on your monitor will usually be less 'severe' when you print it out in small sizes(4R).