Do I have to defrost chicken before cooking?
Not a great idea. The oven cooks from the outside in, so by the time your chicken is done on the outside, it'll still be uncooked on the inside. If you cook it longer so it cooks through, the outside will be as tough as shoe leather and the inside will be just lukewarm enough to encourage microorganisms (that may have survived the freezer) to fester, which could sicken you.

Put it in the pan the night before, cover it (or not, as Alton Brown sometimes suggests), and put the pan in the refrigerator. When you get home from work or whatever, preheat the oven to 450 F, season it, and bake until done. When is it done? It's done when:


  The chicken is done when an instant-read thermometer reads 165 degrees
  F when inserted into the thickest part of the thigh (the legs of the
  chicken should wiggle easily from the sockets too.) Remove the chicken
  to a platter and let stand for 10 minutes, so the juices settle back
  into the meat before carving.


http://www.foodnetwork.com/recipes/tyler-florence/the-ultimate-roast-chicken-recipe/index.html

Note that some would say 165 F is over done, but safe is better than losing a kidney to E. coli or other nasty microbeasties.