@OP: Assuming you have checked the entries inside the router, type "ipconfig /release" and then "ipconfig /renew" to flush the address and force it to get a new address. Failing that, set a static IP on the XP machine within the IP pool of the router and check for connection out. Lastly, are your wifi drivers up to date? Plug your XP machine into the router via a network cable and check if you can gain access through there to determine if it is wifi specific problem or if it is a machine/operating system problem.
I found that sometimes when I have been travelling and I have a number of different wifi points listed on my laptop, I have problems connecting to another spot. I just tend to go into my wireless networks under wireless network connection properties and delete all my preferred networks out and then try and reconnect to the new network.
I usually find it helps me. I am also using Windows XP service pack 3
I've got to stop saying "how stupid can you be?"...........Too many people are taking it as a challenge.
basicly run cmd and type in ipconfig and check if everything looks fine. then try and ping all of the computers ip numbers. you also need to check firewall settings in windows 7. go to control panel / windows firewall / advanced settings / windows firewall settings / protected networks. then untick you lan / wireless networks - otherwise it will bock everthing, and it will give alot of problems. also make sure of internet sharing setting. ip normally is 192.168.0.1 / 255.255.255.0 on the host computers. so make sure all the other computers are running on the same subnet mask. if you need additional support , contact me . regards, metrohub.