This is a very common problem on side pull caliper brakes.

There are a few things you can do. Apply grease to where the spring interacts with the brake arms. Adjust the spring with a cone wrench or the special mentioned above. Loosen the brake where it attaches to the fork, recenter it, and re-tighten it. Some combination there of will do the trick.

More information here: http://www.parktool.com/blog/repair-...-brake-service