Let me warn you first that if you do too thorough a job of correcting the problem of squeaky floors, the problem may return because the squeaking was caused by expansion and contraction. With that in mind, you should only make an effort to fix sections of the floor that are exposed to a lot of traffic and let the rest move. Have someone walk across the effected areas while you touch the same floor from the basement. If you feel the floor move and hear a squeak, gently insert a cedar shim with a small amount of glue between the floor and the joist. If you cannot feel any movement, the problem is between the subfloor and the hardwood. Screws from below can pull the hardwood down securely.