HELP! HELP INTERNET CONNECTION SHARING

I build up small LAN using a xp computer to do dialup to internet when ever each client request. As I setup the each computer, it automatically assigned an IP address which I prefer. After 6 months the network seemed to be bogged down with traffic and slows down the downloading of email and internet surfing
What causes this???? and how can I solve this problem.

After I review each individual IP address some of them have been changed from the initial IP which was automatically assigned during setup. What causes this and how can I solve this problem.