Thatīs strange...

mg2:
Thatīs the first time, I hear, that re-installing a game causes so much trouble! :blink:
I just installed Diablo 1 on my PC (with Win XP). It works without a problem; it loads the patch and I can connect to the battle.net without any problems.
Which OS do you use? I figured out, that the game has some problems, if the compatibility-mode (win 95 or Win 98/Me) is activated. (atleast on my PC. Maybe you should try out the compatibility mode, if you havenīt tested it already.)
The following may seem to be silly questions, but often the most obvious things get overlooked

:
When you are at the multiplayer-menue and you can choose the type of connection: Is the option "Battle.net" activated, before you try to connect?
Which Battle.net-Gateway is set? Try to connect to another gateway.
Well, at the moment thatīs all I can think of. I would suggest, that you visit the blizzard-site and some Diablo-Fansites.
I only know german fansites; so I donīt think, these will help you. But I found something in the Diablo-FAQ at the Blizzard Support. Maybe
THIS is helpfull.
Edit: CD-Key? Diablo1 has no CD-key. You can play both Singleplayer and Multiplayer without it.