Hey Phil,
That's the correct IP address for this server?
If it is not the correct IP address, then do an
ipconfig /flushdns from the command prompt.
Otherwise, try using an internet DNS server, try nslookup with
www.pchelpforum.com IP: 64.118.92.21?
If you can not resolve any DNS servers, then check the firewall on the local PC.
If you can resolve other DNS servers, but not your own, then make sure there are no firewalls between the PC and the server. You could always plug that laptop into the same connection as an existing PC. It's possible that this problem could be with the server itself, has it been restarted? Other PC's may be working by using cached DNS, but I believe flusing the DNS could also be used on the other PC's to determine that.
One way around the error when it comes to joining this to the domain, is by adding the IP address of the DNS server in manually? It will still pickup a DHCP IP address and subnet.