Hmm, some suggestions...take them as you will.
First, it might help to know what type of hookup you are on.
If you are on a dial-up it could just be multiple timeouts with the config you have currently.
If you are on a DSL check to see if they have a firewall as well. In fact, I believe XP and DSL could actually give you double firewalls, which would mean you have some serious stuff to work on.
On Cable...hmmm, now comes the next part.
Can you hook up with any of the other games that you possess?
Can you hook up to Gamespy and it's matching service.
If you can do that, next, do you have a Gamespy account, and do you have a password.
Try connecting to some matches there, if that doesn't work, then it shows that something is up with either your account, or the person to person hookups.
In that case, you may have problems with ICQ and other chat programs as well, which I expect you may be having as of late since your config was messed up.
If you can match up there and get in game matches, then reasonably you should be able to hook up to the D3...unless you have a different game version for some reason I can't fathom.
Hope something here helps you.