Carpenter Ants are probably one of the most home damaging insects in Canada and one of the most difficult to control. They don't eat the lumber in the walls of your house; they just dig tunnels through it to make a comfortable living space, but that tunnelling can destroy the structural integrity of your walls. They generally travel outside the house to find food and carry it back to the house. Sometimes the primary nest can be in your walls, sometimes it is in a log outdoors and you are just a satellite nest, meaning that even if you kill all the ants in your wall you may not have gotten the queen.
PRO OR DIY?
I am sorry to say that I agree with the professional exterminators who will tell you that most efforts at killing the ants in a DIY fashion will probably fail. The most useful thing you can do to discourage the ants is to move firewood, overhanging branches and such things away from the house -- but even with those precautions, these hard working little bugs can go over 300 feet from the house looking for food. Non-toxic diatomaceous earth sprinkled around the house on a regular basis can block their path on the ground, but inspect for ants running along electrical wires as well.
IDENTIFY THE ENEMY
It is a good idea before panicking too much to know if you really have carpenter ants or some other type of ant before going too far, certainly before you start ripping open walls. Start by trapping one of the ants and then take a close look at it with a magnifying glass. The University of California has a great little interactive web site that will allow you to Identify your Ant. If the result of this exercise is that you do indeed have Carpenter ants I would recommend that you visit a very informative site set up by Pest Control Canada.
People constantly complain that they get different extermination plans from different "professionals". It is true that there are several possible ways to attack the problem. The best thing I can recommend is that, as with all contractors, get references from clients who had the same problem and call those references to see just how it worked out. That will help you to feel confident that you have located a contractor who really can solve your problem.
WHAT YOU CAN DO YOURSELF
For a compilation of homeowner control measures insects in general, follow this link to Controlling Insects.