Ohhhh, I do remember having horrible, horrible problems getting D2 to work over LAN. It worked, in the end, IIRC...But I sort of lost all the hoops we had to jump through.
Anyway, first thing to do is turn off everything that in any way might stop anyone from connecting to either computer. That means all firewalls, virus checkers, yadayadas. Get off the internet before you do so :-P
Make sure the pcs can see each other correctly, and completely, and make sure it sees the other one with the right IP number (it wasn't D2, but I remember a game where while my pcs both had working IP numbers, they *saw* each other with completely different numbers...And no, they didn't both just have 127.0.0.1 or something
. I never understood how thatgame came up with those numbers ,but eh, they worked).
Anyway, other than that most basic bit of nonsense, I can't really help. I'm decent with PCs, but I absolutely suck at networks.