This behavior is quite normal, but its not DHCP causing your problem its WINS. Windows clients that recieve an IP address from a DHCP offer, can also recieve a WINS server address from the DHCP server. WINS works much like DDNS, in that once the client gets its IP it will then attempt to register its (NetBIOS) name with its WINS server. The WINS server maintains a database like DNS and then resolves names to IPs Machines shutdown cleanly will actually 'release' their names, but if its a laptop, the user often just suspends and disconnects from the network. This will leave the name registered. The default period for it to hold the name (the 'release interval') is 6 days. So once the host appears elsewhere on the network, WINS will still resolve to the old location. The new name may be refused by the WINS server (the host will retry every 10 minutes), of course the name may also be accepted by another WINS server on the network but then you get WINS replication playing havoc (Some have the new address, some dont) and all sorts of silliness. Basically, you need to reconfigure the 'release interval' for WINS on the WINS server(s), under intervals in the WINS admin snap.

-Maestr0