Here are three free ways to correct barrel distortion, and two to buy:
Panaroma Tools is the best free plugin for Photoshop, Paintshop Pro, etc.,
contrary to it's name it also corrects barrel distortion. http://www.fh-furtwangen.de/~dersch/
PhotoGenetics can de-warp a picture, but beware of it's jpeg save, it makes the file a lot smaller, so it must be throwing away data. I haven't used it enough to know all it's quirks yet. http://www.qbeo.com/
A free full version of Photogenics 2.0 can be downloaded for signing up at http://www.printroom.com