Not sure if a real techie would agree with this, but how about a small soft paintbrush, or something like that? You can even (VERY carefully!!) use a hoover, with a small ended attachment, to clear out the dust residue from the bottom of the case, once you have brushed round. Make sure you wear an anti static wrist band, or at least earth yourself on a radiator or something, before poking about inside the case, and obviously make sure the power is disconnected. I'm only saying this, because it's a method I have sucessfully used myself. If the fan is that bad, it may be preferable to replace it, but you may be lucky and cleaning might work.