try to access your router setup homepage from the non-working pc. it should be an address like 192.168.1.1 or 192.168.2.1, but some routers use diff ip addys
when you can see ur DHCP clients list on one of ur pcs u can see if you are actually making a good connection
if your non-working pc has a valid DHCP client lease then it is in fact connected, and something in ur xp is blocking you
also you may try doing a hard reset on your router, usually a small pinhole on the back of the router like the pinhole on a dvd tray. press that for 15-30 secs to completely rest everything to factory defaults if nothing else works