Simple really. Just make sure the two are connected together either by an Ethernet cable (make sure it's a crossover cable if it's just two computers connected directly). iTunes (for PC) has technology that comes bog standard with Macs called Bonjour which automatically configs the network. Simply turn on sharing in the PC (Edit > Preferences > Sharing > Tick box > Click OK) and make sure the Mac is looking for shared music (iTunes menu > Preferences > Sharing > Tick box > Click OK). The music from the each computer should appear in the left hand column which has Library, Music Store and all your playlists in it....
Job done...
However, if you want to have it so all computers share a central iTunes library (like I do, except for my MBP), it's quite different (very different) to that....
I shall explain that if you want me to...