Dude that is the simplest network setup ever
hock up your router and DSL modem and server like this
from out side it:
Internet<--- Cable Modem------>WAN port of Linksys Router-------LAN Port of Linksys----NT server
If you had a network going before with few computers, then that means your NT box was doing the DHCP for you,, you may want to hand that to the Router,, further,, you can us the LAN port of your Router to connect it to another switch or connect your other system to it if you have enough port on the router,,, check out all your client system to make sure that the don't force IP address, and if you run WINS server, disable it.
There are tones on web sites which would show you diagram of simple network setup.
cheers