Sometimes it pays to give it time.
You don't say who your ISP is; if it's BT, the reason may be that in the last few days they have been extraordinarily inconsistent. It is quite possible to have a 'phone connection and a good handshake with user name and password and then get nowhere.
Sometimes with BTInternet you can get their homepage but not their pop mail server; and sometimes vice versa.
Don't always suspect your setup or your procedures; it could be a situation outside your control.
If you did a clean Win2000 install you should be Okay. I'm very impressed with what I've seen so far.