How do add songs from itunes to my iphone when I have a new PC

I recently bought a secondhand PC, and installed itunes and uploaded all my music into it from an external drive. I've plugged my iphone in, but I can't add anything else to it, and it seems the only way is to select Sync music, which says it will wipe out all music, film etc if I do that! Is that the only way? It's not really fair is it, as that's my music that I have uploaded with my own CDs and I don't really want to have to do all my playlists again!