you're connecting to the internet via the wireless USB connection on the XP computer and this connection is shared?
manually configure the LAN connection TCP/IP settings on the XP computer:
IP 192.168.0.1 subnet mask: 255.255.255.0 no gateway, no DNS
now configure your router (use the Vista computer, the XP machine cannot connect to the router now) for static WAN
IP 192.168.0.2 Gateway: 192.168.0.1
the trick is: not the Vista computer, the router has to get the internet connection from the XP computer, then you can access the internet through the router from the Vista computer. thus you have to make sure that your XP computer is connected to the WAN port of your router.
however, if you want to share files between the 2 computers, you'll need a second NIC for the XP computer to connect to a LAN port in the router.
alternatively, do away with the router and get a crossover network cable (Vista computer either to automatic IP or 192.168.0.2, subnet 255.255.255.0, gateway and DNS 192.168.0.1). now can have both, internet connection AND file/printer sharing.