DHCP Relay cannot be run on a server that is running DHCP or NAT with automatic addressing enabled
You need to have routing and remote access services installed on your server, then you go into the routing and remote access control panel and go to IP routing >
Right click on general > add > select DHCP relay agent and then configure the interface to relay from, tick to enable DHCP relay, specify the max number of jumps etc.
Once you have the relay set up, right click on it and choose properties, you can then specify where the DHCP requests will be relayed to.
Some guides that may help:
Microsoft Technet guide: http://technet.microsoft.com/en-us/library/cc771390.aspx
Diagrams that may help:
You may want to consider using routers with built-in ability to forward dhcp requests, or put in some sort of VPN