How to set up a static IP address on a Windows XP computer
It is very important to setup a static ip address, if you are going to use port forwarding. When you have port forwarding setup, your router forwards ports to an ip address that you specify. This will probably work when you initially set it up, but after restarting your computer it may get a different ip address. When this happens the ports will no longer be forwarded to your computer's ip address. So the port forwarding configuration will not work.
What is an ip address?
IP addresses are four sets of numbers separated by periods that allow computers to identify each other. Every computer has at least one ip address, and two computers should never have the same ip address. If they do, neither of them will be able to connect to the internet. There is a lot of information at the following link. You don't need all of it. But if you want to know more about how networks work, you'll find it there.
Dynamic vs Static IPs Most routers assign dynamic IP addresses by default. They do this because dynamic ip address networks require no configuration. The end user can simply plug their computer in, and their network will work. When ip addresses are assigned dynamically, the router is the one that assigns them. Every time a computer reboots it asks the router for an ip address. The router then hands it an ip address that has not already been handed out to another computer. This is important to note. When you set your computer to a static ip address, the router does not know that a computer is using that ip address. So the very same ip address may be handed to another computer later, and that will prevent both computers from connecting to the internet. So when you asign a static IP addresses, it's important to assign an IP address that will not be handed out to other computers by the dynamic IP address server. The dynamic IP address server is generally refered to as the dhcp server.
for more detail look at http://portforward.com/networking/static-xp.htm.