Dragon, sometimes IP configurations can change if a certain system isn't powered on when others are. The reason is because one system stores all the information about a network and workgroups and keeps a table of what systems have what IPs. If that system isn't on when another boots up, the other system may set its own IP at the very initial IP of the set IP range. If you need to avoid this, disable DHCP and configure the IP of each system individually and manually. They will not change again, unless you change them.
Simulaire, make sure the router is a NAT (preferably with SPI). NAT is the single biggest security enhancement for the consumer market.