Of course Henry is right about the products to use to get the old adhesive off, but the tar type adhesive might be too old and may be too much in abundance for a chemical remover (depending on how thick). You migh want to consider having them (or doing yourself)sanded down in order to obtain the optimum results of your hardwood floor. This way you can refinish with the stain or finish that YOU prefer rather than the look that exists or that could be old and worn in high traffic areas. Just a thought for you to consider.