Router and Server 2003 setup question??

I have got a local area network with about 30 clients with an AD dc running dns and dhcp with exchange.
There is a cisco router on the lan which goes out to a company wan and the internet.
We used to have an alternate internet connection locally until the modem died one day.
I have been researching on the internet about the configuration needed to set this up and have been not very successful.
Can anybody point me in the right direction as regards to Server dns and dhcp config and also what needs to be disabled on the router.

many thanks.