XP pro assigning IP addresses

We have a 120 user netware network with almost everyone running Win98. One employee is using XP professional. We use static IP addresses, but people sometimes have theres set to auto obtain. I have noticed two incidents where computers set on auto obtain are using the XP pro computer as a gateway and stealing static IP addresses. Somehow, the XP pro machine is acting as a DHCP server. Does anyone know what this could be and how to stop it?