You don't need a special tool to remove old grout, but you do need the determination to do it right.
When regrouting a ceramic-tile wall or floor, the first step is to remove as much of the old grout as possible. Although there are special grout saws available at most hardware stores, the pointed tip of a lever-type can opener often works just as well. Draw the point repeatedly along the joint to scrape out the hardened grout. If the tool becomes dull, clamp it in a vise and resharpen the tip with a file.