What is the best way to heat an enclosed porch?

We bought a home which has an inclosed porch. The door side also has windows, meaning just two storms and a screen for each one (not regular windows). One side wall is against a regular room in the house, but has a regular window. The other side wall is all storm-screen windows. The fourth side going into the house is another door, and a regular window looking into the living room. The owners had put a thin layer of insulating styrofoam, before covering all non-window surfaces with vinyl siding. This room (porch), despite having plastic on all windows, is still freezing. What would be the best way to heat it? Can more insulation be put in the siding? Is it a must to spend thousands for regular windows?