I had a problem getting games to launch like this a while back and posted it, never got a response though.
my LAN is configured properly, we play many multiplayer games with out any problems at all. But i fire up AH and try it and no dice. I can see the host game and when you select it you get a CTD and no error.
Also i shold note that if i host a online H2H game that no other PC on my network can join the game, same deal as above.
very odd i think
The only thing that i can think of is the router, but NO other game has problems with it. I have the latest firmware for it and its running just fine.
i can run multiplayer:
quake III arena, rocket arena
age of empires II conqueors
empire earth
battle relms
SWAT III
return to castle wolfenstein
ghost recon
(more i cannot think of off the top of my head)
I can also file share on the network.
both machines has windows XP installed and nearly identical hardware. if you need the specs let me know, i honestly dont think that the pc's hardware is the culprit, we've tried 3 different PC's and no deal.