Walk over the floor slowly, in courses, setting your weight down deliberately and listening for where the squeaks are. Put a mark with your piece of chalk at each point where you hear a squeak. (Note: If the squeak seems to be right on a seam between two floorboards, don't put the mark on the seam, but put two marks on the boards next to the seam, each about an inch from it.)
Load your power drill with its pilot bit. Walk the floor again, sinking holes straight down into the wood at each point where you put a chalk mark.
Take out the pilot bit from your drill and put in the screwdriver bit. Sink wood screws straight down into each pilot hole that you drilled. Drive them deep enough that the heads of the screws are slightly below the surface of the wood.
Use your putty knife to press wood putty into each screw hole, covering the screw head. Press the putty in tightly, and get it smooth and flat. Let it dry for four or more hours.
Buff the surface of each spot of putty with your sandpaper, getting it completely smooth. Brush on a little polyurethane. Let it set for a day.