I have an odd problem. My friend and I have almost identical systems. We seem unable to connect to each other. When he or I host a Killing Floor server, it shows up as "not responding" and won't even appear in the server browser. When he tries to join my Minecraft server, the server lags like mad, he gets disconnected, and his internet disconnects, he has to unplug and plug back in before it works again. Its just he and I, too, I have played with/hosted with plenty of other people.
So what's the deal?
When he first got his computer I had to make a network bridge to give him internet because he didn't have a wireless adapter and there are no hardlines in my house, but the bridges on both our computers are deleted. Just thought I'd mention that.
edit: I'm running Windows 7 64-bit, he is running Windows XP 64-bit
Firewalls are off, and yes we can ping each other. The problem seems isolated to Minecraft and Killing Floor, its common in Minecraft and as of yet there seems to be no solution. Some conflict between XP and 7.