Lowering the ISO speed will make pictures less grainy, but not as bright. You can compensate by increasing the shutter speed and/or lowering the f/stop if that camera has the capability. My old Canon Powershot A40 has it in a limited fashion, so maybe yours does too.
Lowering the f/stop makes the aperture open more, so more light is allowed in. It also increases the depth of field.
http://www.dcresource.com/reviews/ca..._sd630-review/
this review has a bit about shooting at in low light conditions.