The DNS of your ISP does the trick converting names to TCP/IP numeric addresses. Apparently you have entered the Domain Name Server's address in the otherr computers, but not in the one with problem. Check either of the other 4 computer setups and make the fifth identical.
It's also possible that your router supports only 4 clients.
Running MS XP Home on 5 PCs on home network. All have Office 03 and current Norton AntiVirus and Firewall installed. One computer can not access internet via 'www.xxxxx.com' but works with 'IP' address. Example: www.yahoo.com results in 'cannot find server' message...but entering 126.96.36.199 works just fine.
? Where is the 'alias lookup' file that converts 'www.xxxxx.com' entries to the 99.999.999.99 IP adress? And how do I correct the problem?