not sure that removing dust would drop the temp that much! depends how dusty it is i suppose! it sounds stupid but i hoovered out my pc and dropped the temp by 5 or 6 degrees, as it was insulating the motherboard and blocking the case fan exhaust. if your pc is quite old or just very dusty you could also try taking the fans apart and cleaning the spindle/bearings (depending on how much you can pull it apart before you can't put it back together again!). they seem to act a bit like candy floss machines and whip up dust into a clump which can slow the fan down considerably and so increase the temp in your case. This can be especially important on your CPU fan.