Wayne Grudem has an excellent summary of the major differences between Protestants and Roman Catholics. He produced it as a class handout for a seminary course. It is online here and reproduced in entirety below. (Wayne is a seminary professor and evangelical theologian; his web site is here.)
Numbers refer to pages in Wayne Grudem, Systematic Theology (Zondervan, 1994) and to paragraphs in Catechism of the Catholic Church (Ignatius Press, 1994).