change ftom static ip's to dynamic ip's

I am running Windows 2000 Server and 15 clients on one network. I have a router w/ ISDN connection to the internet. All of the clients and the router and the server have a static ip address. Our ISP is not going to cover the area I'm in anymore and to purchase a block of 20 ip addresses from another service provider is to expensive in our area. How do I go about changing my server over to assign dynamic IP addresses to my client computers, and have the client computers and server still access the internet through the router. My new ISP will assign us 5 static addresses, so I can use 1 for the routher and 1 for the server.
Thanks in advance