If there is already a pre-existing DHCP server on the network, do the following:
Take one of the workstations, disconnect it from the network and plug it directly into the Router. Make sure the Router is turned on and there is a link light corresponding with that machine.
On a single LAN port router, hook your computer into the LAN port with a straight-through cable and set the Router's toggle switch to 'X'.
On a multiport router, plug the workstation into any port except the Uplink or WAN port.
Reboot the computer. Check the IP address and make sure it is within the router segment of 192.168.1.x.
Then access the Router's web based utility at http://192.168.1.1. 
You will be prompted for a User Name and Password. By default there is no User Name and the Password is admin.
This will bring up the Setup page. 
Click on the DHCP tab. This will bring up the DHCP section.
In the “DHCP Server” area, click on the Disable option and click on Apply.
Click on the Setup tab.
Change the LAN IP Address on the router to be on the same Subnet (AKA “IP Scheme”) as the rest of your network, but be careful that and make sure it is a unique IP Address, outside of your DHCP Servers IP Scope.  If your network has a different subnet mask, it must be class C,(if it’s not the router will not work) and adjust the routers LAN Subnet Mask to match your networks.
Click Apply
Restart your computer.
After you’ve completed these steps, you’ll have to configure your DHCP server to give out a default gateway, which corresponds with the routers LAN IP Address.
Once you have completed these steps your DHCP Server should work with the router.