NIC s do not work in ACER (sis 620) WIN 2000 spanish

I have two Acer S Series desktop computers. One is upgraded from win98, the
other is a fresh instalation of Win2000 Pro.
The Network Interfase card is detected and the driver installed. The
computer(s) respond when pinging the localhost or it´s own IP (or
computername), but the computer (s) can't communicate with the default
gateway or any other host in the network.
I have tested and changed cables; I have tested with 3 different NICs
(Genius, Realtek 8139 and Realtek 8029). Results are the same. The most
recent drivers from vendors were installed too and no advance.
The same network cards work ok in other CPUs.

Any idea?
Thanks in advance
Jianwa.