In Reader to Reader: "DHCP Conflict" (February 2001), Melissa Wise describes a common situation on our LAN. We have several users who are out of the office for a long time (i.e., their computers are shut down for a period longer than the lease duration specified in the DHCP scope on the server), and the server will often lease their IP address to another machine. Thus, the systems might not be able to release or renew their leased IP address if the server has leased their IP address to other machines.
The following alternative to Melissa's solution doesn't require registry editing. On the DHCP server, make a reservation for the out-of-office user's computer by using the system's media access control (MAC) address and any available IP address. Reboot the computer. The reservation will force the client system to take whatever IP address you reserved for it. Next, delete this lease on the DHCP server. Reboot the client computer. The client system will take the available IP address from the subnet scope and act like a typical DHCP client.