I have a PC/MacBook setup. I have a decent sound card installed in my PC and therefore, the speakers are connected to that.
But I want to use the PC connected speakers to play music through AirTunes – through my MacBook or iPhone.
Can it be done?
If you use iTunes 10 or later (either on PC or Mac) you will see the AirPort Express' Speakers as AirPlay option in your iTunes.
If you want Windows to be the "AirPlay Target" so that you can play your music from iPhone over the Windows-Speakers you could use Shairport4w.
Shairport4w will requires Apple Bonjour which is installed with iTunes or you download the Bonjour Print Services for Windows OS if you don't have iTunes on your Windows machine.
There's a decent sound card in your macbook, it may just be simpler to use that. If it's a physical location thing, I make the playlists on a mac and share the playlist to a windows computer and connect that to the speakers.