Connectivity problem

I upgraded, apparently successfully, an NT4 PC to XP Pro. The problem is I’m unable to connect to the Internet or join a domain. It appears to be a DNS problem (error trying to join a domain = DNS name does not exist) but nothing I’ve tried works. I can ping internal and external IPs fine. I’ve used DNS IPs that work with other PCs. I’ve run the XP network troubleshooting utility and it says all is OK. I’ve searched the MS KB with no luck. Help!