do you have itunes on the mac set up to search for shared music (somewhere in prefs), and have you checked your firewall?
Yes, the option to search for shared music is enabled, and I disabled the firewall to be sure it was not causing the problem… 🙁
Is the mac wireless instead of wired, as compared to the other two pcs?
If so, can you check your wireless accesspoint/router and see if it has an option to enable multicast or IGMP? If it is wireless, can you test it briefly wired and see if works that way?