I'll bet the iPod is formatted for Mac and you manually manage your songs in which case the easiest way to do this would be to copy your songs back onto your iBook using
Senuti.
Then, plug your iPod into the PC and format it using the
iPod Updater. This will erase everything on it and return it to pretty much exactly how it was when you bought it.
Then, Plug it back into the Mac, enable it for disk usage through the iTunes preferences but don't get it to sync libraries (you'll be asked) and drag your music files onto the iPod icon on your desktop.
Then, plug it into the PC and drag the music into the PC's version of iTunes. Then, sync the libraries or drag them across (if you want to manually manage songs again).
Notes: reformatting your iPod will lose
all data on it. However, a PC formatted iPod can work just as well with a Mac or PC, the opposite is not true. If your iPod is already formatted for PC, then simply plug it into the PC and (once you've enabled the viewing of hidden files through Explorer) drag your songs into the PC's iTunes library. Don't let it sync when it asks though. Also, one last thing, this doesn't take into account purchased music from iTMS. For more info on that, check out
this link about deauthorising your computer and check out
this better description by Apple on how to use your iPod to transfer tunes between PCs too.
