The problem here is that you ARE trying to spot reduce. Some people carry fat in their legs, others carry it in a "spare tire" fashion, or on the backs of their arms. To reduce the fat in your legs you must reduce your total bodyfat percentage, which will come from a healthy diet and cardio.
However, it is good to build up the leg muscles, because then you attack the problem from two angles-- you build up attractive muscle underneath with squats/deadlifts/etc. and you reduce the fat on top through cardio/eating properly.