If in the iTunes options you've set iTunes to manages the music files itself then all your songs & playlists are stored in the iTunes music folder (generally %UserProfile%\My Documents\My Music\iTunes Music unless you've changed that as well in the settings). You just need to copy that folder over to the other PC & then add that folder to the iTunes library.
If you don't want to go through that hassle, you can just enable library sharing on the local network. It's very easy to turn on & use.