You can change the IP on Windows 7 from DHCP to a static IP. With a static IP you need to provide the IP, the net mask, the gateway and the DNS servers. If you leave the gateway field blank, then Windows 7 can only reach IP addresses in its own network. If you put the router's IP into the gateway field, then Windows 7 will send all packets for IPs which are not in its own network to your router and the router will do the routing into the internet.
Read your router's manual to check which IP addresses are free to use and which are reserved for the routers DHCP server.