Just like with my likeness, I used Microsoft Expression Design 2. It is part of a suite that I use at work. If you are just starting out, try Inkscape as it is free http://www.inkscape.org/. Review the post: The New Me (as a Cartoon).
I like vector graphics programs for doing cartoons because you can start with a small grainy photo and once you have “traced” it, you can enlarge the vector image to the size that you want and not get distortion.