Using an ICA Client in Windows XP

I set up a new network in an office. An existing Win 2000 Pro machine and 3 Win XP Pro machines. Connection to the Internet (ADSL) is via a Nortel Contivity 100S box. The Win 2000 machine can connect to a Citrix server via the ICA client. On the XP macxhines, the ICA client returns and error message that “network or dialup problems are preventing communication with the Citrix server”. I checked settings for the TCP/IP properties of the NIC’s in the Win 2000 vs the XP machines and there are no differences. Any thoughts, anyone?