I think Chessmaster is the most user-friendly program. It's pretty much designed for you to play and learn against the computer.
Fritz and Rybka are stronger, which is useful if you're trying to prepare against a tactical opening, especially for top-level chess. These two engines use the same interface.
which program should i buy if i really want to learn chess not just play a powerful engine? fritz 12 rybka or chessmaster11. what are the pros and cons of buying the software.