1. enable Firewire network device on PC
2. enable ICS in WinXP and use Firewire network connection as the proxy. This is important because the XP machine will act as the DHCP server.
3. Setup firewire network interface in OS X
4. Boot PC first
5. Boot Mac - should pick up an IP on the firewire interface given from PC, if not, check all settings and repeat.