DHCP Setup

Our company is in a building with an available network on which a DHCP is running. We don’t need this DHCP but we can not disable it as it is not under our supervision!
Untill now our own network have been using Static IPs. We would like to start using a new DHCP in our own network while the other DHCP is running meanwhile.
I setuped our own DHCP server but the clients are getting their IPs from the other DHCP server. What can I do to fix this problem? (I am using windows 2003 Enterprise Edition-SP2 and a DNS server is running on this server too)