First are you connecting the the AS/400 name or IP address. Which ever it is try Pinging it. if that is ok. Then goto a command prompt and try to TELNET into the as/400 the command would be TELNET “IP Address or computer name” without the quotes and plug in your as/400 computer name or IP address. if that all goes well then try a newwer version of Client Access I believe some older versions have problems with XP.
If Pinging doesn’t work and you are using the AS/400 name edit your host file or start using the IP address instead. if none of that works let me know the results and I will try to help you more