Can you plug the Windows-formatted iPod into the Mac and simply grab the music from there? The Mac and the automatically-syncing Mac-formatted iPod should have the same library (or at least some identical playlists depending on how you sync the iPod).
Otherwise, we may need more information before we can get any further. Hopefully you have the rights to all this music because otherwise MacRumors won't be able to help you.
if you have a Mac handy then the easiest way is to plug the iPod into your Mac, copy the music off it by using [Guide]Senuti[/Guide] into iTunes then using iTunes sync the music back to your Windows iPod, if you don't have a Mac handy and do have a Windows PC you can use XPlay to copy the music, it has a free trial but costs $29.95 otherwise.