It's because your iPod can only be synced to one computer at a time unless you enable the Manually manage music and videos option from under your iPod's summary tab. However, this method of syncing is much different from having iTunes automatically sync content for you.So in order to sync your iPod with the new iTunes you'll need to copy the music from your iPod over to your new PC and then erase and sync it.
' pwede nmang e drag ang music kung saan mo gusto ilagay dun sa ipod mo
' yun nman ang ginagawa ko... pero kailngan may itunes ka na nka install... at e open mo ...then saka mo e drag... may apple Ipod yata ako orig at sonywalkman Mp3 player..
nyahaha hindi dinadrag yon..