In your /Applications/Utilities/, there should be a folder called "iPod Software Updater," containing one or more software updaters. I guess the problem is that you may not have one for 1.0 ... try running the earliest one you have and see what version software for the iPod you have it has on it. Usually, if you want to flash an old version of the software, I think you can use a past updater to reformat the iPod, but if you want to go back to 1.0, I'm not so sure....