TCP/IP Connection Continual Failure

Greetings.

For years my friend and I have played games back and forth without much issue. However, one game, Age of Empires II w/ Expansion, continually gives us fits. It's the only game that fails to work for us, yet it has intermittently over the years, on sheer blind luck.

I'm not a network pro, but I'm not a novice, able to successfully navigate mine and other's routers/network problems before. That said, I still know squat compared to everyone else, and more help the merrier. I've built computers for years, and I'm a fairly confident person to find my way around. With that said, let's get to the issue.

One friend and myself, TCP/IP connection. Simple enough, right? Nope. Fails with AOE though. Don't know why, and I'm really incredibly sick of triple checking settings and being clueless as to why it's not working.

I have Vista 64-bit Premium, he has XP Pro SP2

First off, I've got easy access to both our computers, routers, and home networks. As the game calls for, I've got the DMZ host open on both routers, disabled blocking WAN requests, and enabled the ports for 47624 and 2300-2400 for both TCP/UDP, for both our computers network address. It's worked before, never had problems with other games // sometimes Age of Empires. Even when I used to XPSp2.

If I use direct connections, bypassing the routers, turning off our simple Windows Firewalls, still no connection. We both obtain our IP addresses automatically, though notably my friend has to manually set a DNS when he's connected to a router.

We recently tried Igzone as the new MSN place to play games, to test it out from frustration. I was able to access the hosted game, my friend was not. So it "seems" it's his computer?

I can ping his IP address and get a response, though if him or anyone pings mine, it times out. An issue on my end?

That's about all the information I can think of/know of/can provide. I've tried about everything, and games have run flawlessly (with tinkering) through routers/firewall before, but this time, even disabled, I'm running into fits. Are there hidden network settings into Windows XP/64? Something with the DNS/gateway? I've got not a clue, and I've had great luck on these forums before. Hope you guys can shed some light on the situation.

Thank you for any help.