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.
For more information on copying content to an iPod manually see this article how to transfer or copy music from iPod to PC.
Hopefully this works for all of you like it just worked for me!