Your iPod is designed to sync with only one iTunes library at a time. It will recognize the iTunes library on the PC as a new library. If you sync the iPod with this new library, all content will be erased from the iPod and replaced with what is in the new library. So what you will want to do is copy everything from the iPod to your new iTunes library on your PC first.
See this excellent user tip from another forum member turingtest2 outlining the different methods and software available to help you copy content from your iPod back to your PC and into iTunes.