what is the routers security mode set as?
is it WEP, WPA, or WPA-PSK or something else? as far as I know this is all xp supports if its something else like WPA2 or something then xp wont support it as far as I know and you will have to change your security method to one of these three or run it disabled.
that's where i luck out i don't have another home within 400 feet of my home so i leave it disabled. my wifi doesn't even reach the 100 feet to my fence from my home. so in my case only people up by my home can steal my wifi but then they would be trespassing and out would come my gun