In that case, the iPod needs to be able to show up on both the Mac and the PC. If it shows up on both, then you just need to make sure you can add files to it from both the Mac and the PC. If you weren't able to add files to it, then it'll have to be reformatted into something both computers can use (FAT32 is a good choice).
Then, just take the files off the iPod and put them onto your PC. Once again, this isn't the most efficient way to backup (Time Machine would be your best choice), but whatever is necessary, I suppose.