[idi banashapan]

open a command prompt
type CMD
type ipconfig /all
this will list all the network connections and their current status. does your NIC show an address starting with 169.254? If so, this means that your XP machine it unable to obtain an IP address from the router (which is assumably dishing out addresses via DHCP). being that the Vista machine has managed to obtain one (provided you are not assigning static addresses on the clients), then yes, it would appear to be the XP machine.

in which case, right click on My Computer and select Manage. go to Device Manager and remove (uninstall) the NIC from the list. once this has finished (will take about 3 seconds), rescan for hardware changes via the menu options at the top of the window. this will readd the NIC. try connection to the router again and also see if you are getting an IP in your specified range as opposed to an APIPA address (the 169.254 address)

---------- Post added at 19:51 ---------- Previous post was at 19:49 ----------

EDIT: just seen Xpod had said the same thing. all the same, your response didn't indicate that you actually uninstalled the NIC, rather just checked its status...