Two WAN on Windows 2003 Server

I have a Windows 2003 Server with 3 NIC’s in it:
1) Incoming Corp. Internet connection
2) Outgoing to Network hub
3) Not used — Would like to add incoming aDSL

My 2003 server runs AD, and handles the DHCP.

I would like to add DSL to the 3rd NIC and have it as a “back up” conenction, for when a) there are problems with the main connection b) the main connection is over loaded.

Can anyone explain to me how to do this?

Thanks for any help.