DHCP Server not assigning addresses to clients

I have a problem on my SBS 2003 Server.

DHCP on the server
appears to be running fine, but when I scan for DHCP servers on my network
there are none to be found. Using a built in DHCP server on our router or
firewall works fine.

I have noticed in the Advanced tab of DHCP properties under the Bindings
button, that both my inward and outward facing NIC's are listed but neither
of them are checked. If I check the inward facing card and click OK, I am
unable to apply the changes.

Any help would be greatly appreciated.