We played a 3 humans vs. 3 comps newbie-game for 3½ hours, Sunday evening, on ICO : no lag, no crash, absolutely no problem. The host saved the game.
We continued the game on Monday evening for 3¾ hours : no lag, no crash, absolutely no problem. The host saved the game.
We finished the game on Tuesday, early evening, in about 1 hour : no lag, no crash, absolutely no problem.
The 3 players owned 3 very different types of computers.
You have a certain type of computer (AMD/Vista/8800), your friend has another type (Intel/XP/6200).
I don't know what to tell you : based on your info, who can tell ?
Maybe it's Vista. I belong to the Gamers-who-do-not-trust-Vista-for-Gaming cult : so, I have this unfair, sectarian bias against Vista.
Maybe your router is not properly configured : search the forum, there are excellent threads giving advice on how to setup router parameters. (The only thing I did is forward ports 6000 to 6112 (TCP) to my IP address, but I don't know much about routers -- obviously, my router is not blocking the game traffic when I join.)
The best you can do is e-mail a description of your problems (and the minidumps) to support@ironcladgames.com. Based on the info you have just given, I doubt that anyone here can precisely help you out. Sorry.