The creaking is most likely happening where butting board edges are rubbing off one another or where a board shifts up and down on a nail. To eliminate such noise the best solution is to screw down the boards with 45/ 50mm long screws at 200mm centres. In some American carpentry reference books they also recommend that a line of glue is run along each joist before laying the boards, which would be a real belt and braces job id you were starting from scratch. Avoid punching in more nails- you'll only damage the plasterboard ceiling below- possibly causing nail pops on the plasterboard surface. Taking up the entire floor to replace it would be a major job and quite messy. It would only be worth considering if you had planned on redoing the central heating or upgrading the electrics also- to capitalize on having your floors opened up. If the creaking is limited to a few boards/ areas you could try drilling pilot holes and sinking in screws- I'd drive a screw in about 1" (25mm) from every existing nail or at least at 200mm centres. Plasterboard screws (bugle head, black phosphate finish) are cheap and strong making them ideal for this purpose. You could try this in one room and see if there are any improvements in the floors performance.