If they arent too bad, rent an orbital sander and give them a good sanding, recoat with 3 coats of oil-based. The orbital sanders they rent at Lowes/Menards are so easy a caveman could operate them. However they are slow going. I wouldnt recomend using 36 grit either..sure it speeds up the sanding, but the swirls and scratches the 36 grit put in are a Biotch to get out.
If the floors are in rough shape the only other alternative is to rent or have some body use a drum sander. They are not bad to operate but a lot of damage can be done with them + more work is needed around the walls with an edger, if you go the drum sander route.
Post up some pictures!