I think the problem all along was with the Windows XP firewall. Upon closer examination, my router's firewall is set up between the WAN and my LAN and doesn't prevent any internal LAN traffic. I'm not sure why the receiver still wouldn't boot when I turned the XP firewall completely off, though, unless I needed to reboot my laptop for the change to really take effect.

Anyway, these are the ports I opened up in the XP firewall, and everything is working fine now:

2049 TCP, 12075 UDP, 12076 TCP, 12078 TCP, 18678 UDP

The thread wasn't clear about 12077, so I left it out. No problems yet.

Thanks again for the help, y'all.

\m/