I had the problem that VB used the 'wrong' DNS Server to resolve the DNS in my Win2000 Guest, since I have my LAN and a Cisco VPN Adapter.
For some reason VB was using the Cisco VPN DNS Server, which didn't work. I figured that out, since it said that in the VM's Vbox.log file. After I configured the the Win2000 Guest to NOT resolve the DNS automatically but to use the LAN's DNS Server it worked like a charm
Happy END
However VB should definately make the DNS configurable in the VBox GUI, otherwise it is such a pain to figure out where the problem is.