Matt,

The "screws" are what's called the cotter pins, hence the term cottered crank. They are wedges which are tightened with the threaded nuts. They need to be driven out though the side opposite the threads. You will need to find replacements, as the ones you have now are ruined. Once they are removed, the crank arms will slide right off the axle. Beyond that, read Sheldon Brown's article

http://www.sheldonbrown.com/tooltips/cotters.html