New NIC in DHCP server

I have recently installed a new NIC in my NT4 SP6 dhcp server. This server is multi homed with one card haveing an external IP address (the one i changed), and one having an internal address.

Now when clients try to renew their leases they get the error;

Event ID:1004

DHCP IP address lease (IP ADDRESS)for the card with network address (MAC ADDRESS) has been denied.

At the server end the error is;

Event ID 1011

The DHCP server issued a NACK to the client (MAC Address) for theaddress (IP Address) request.

Any help would be greatly appreciated.

NealP