Well, I don't think there is too much of a difference between my router and yours, I have a 2wire 2701 HG-B. The thing with these routers are, you can only port forward one computer. What you need to do is click the firewall button from your main page. It should be on the top
Then you look for "Firewall Settings", enter your password then submit.
Then you should see something like this:
Click "Add a new user-defined application." Fill out the info like so and click "Add Definition":
You will get something like this, and fill it out like I did:
And then hit back, and then you find it on your list, click "Add", except make sure you have it set to the right computer, and then hit "Done"
That should do it, if not just ask.
Also,
Windows XP. Its crap.
No it's not.