The simplist is turning on sharing in the preferences of iTunes in Windows. iTunes will need to be running all the time.
The next more complicated setting would be enabling file sharing in Windows and sharing the music folder. Then in iTunes on the Mac set the Windows library as the default in the preferences. Neither can be set to organize the library folder. When you add or delete a song in one you'll need to replace the library database on the other.
Another option is to setup the free Synctoy for Windows to mirror it's library to the Mac. That way any changes made it Windows will be reflected on the Mac. For this setup you can only make changes to the library and playlists in Windows. Changes made to the Mac would disappear after the sync.
Really with any of these setups you need to designate the Windows desktop as the master computer. Just think of the Air as a player. The desktop takes care of all database management. I wish Apple would make a server version of iTunes that allows clients to connect and modify a global database but they won't.
