Two of them being XP helps The easy solution is run Internet Connection Wizard on both of them, set up Internet Connection Sharing on the modem one and follow the on-screen instructions As for the MacOSX one, I've not used it before, but from what I know about Windows' ICS, you'll probably need to tell it to get it's IP address automatically (use DHCP or something), and tell it to use the IP 192.168.0.1 for both the internet gateway and DNS server, if you can edit them yourself (you may not have to if you're getting the IP automatically).

There is another method where you use static private IPs on your machines, and should work for any OS on a network, but that's slightly harder to do and has more administrative overhead to set up right (although that's what I use on my network ) But try the easy way above first