Open the Network Connections folder, right click the appropriate connection (Local Area Connection or Wireless Network Connection) and select Properties. Highlight the "Internet Protocol (TCP/IP)" entry and click the Properties button. That displays the box in which you can enter your fixed IP address.
You need to enter a unique address in the same range as that of the router itself. For example, if the router's IP address is 192.168.1.1 use something like:
IP address - 192.168.1.5
Subnet Mask - 255.255.255.0
Default Gateway - 192.168.1.1 (the *ROUTER'S* IP address)
DNS Server Address - 192.168.1.1 (the *ROUTER'S* IP address)
Just make sure you are allocating an address in the same range as the router and other PCs (the first three parts or "octets" of the IP address) and that you pick an unused number for the final octet.