Yea, I've been having problem's networking with games, First off all I couldn't get any games to lan beetween xp and win7, then I upgraded the xp mashine to win7 64bit,,,and at first I was having all sorts off problems with lan but as time passed the problems seemed to sort themselvs out.
Updating to windows live (suit of programs and updates including silverlight, messenger ect) seemed to help allthough I have no idea why,,,,
Also you need to make sure that the fire walls on boath computer's have picked up you're game and are allowing it to comunicate,
Maybe some port farwarding on you're router may help also.
Currently I can connect to moast of my games but my network keeps dropping out and kicking me about 10 min's into game!!!! as soon as i find more info I will pass it on!!
good luck!!