Strange DHCP Network Behavior on workgroup

Customer sets up a test network to test our product consisting of one 2003 server and one XP machine.

Both machines are in the same room but connect to each other through a global corporate network with 25,000+ machines installed.

Both are given the same Administrator password and all the work was done under the administrator account.

The server could access the XP machine fine. Ping, RDP, connect to C$, connect in computer management etc all worked ok.

But the XP machine could not see the server. This was the problem. The XP machine couldn't even ping it. Nor its IP address. Ping did resolve the server name to an IP but then the actual pings were timing out.

The server was on a fixed IP address and the XP machine was on DHCP. Both machines were within the same X.Y.Z.mmm 255 IP range. When I moved the server to DHCP, it got a new ip address, and the XP machine could now access the server easily through its IP address. But when you ping the computer name of the server from the XP machine, it was still showing the OLD IP address even though both boxes had been rebooted (and I did an ARP -d *)

Any ideas why this could have happened ?
(a) Why did DHCP'ing the server make it suddenly available through IP ?
(b) Why did the ping continue to resolve to the old IP address ?

thanks