How are these two connected to the wireless router? By RJ45 Cable or Wireless or a combination of both? The problem is more likely to be the wireless router not issuing the IP addresses correctly.
Best solution is to turn off both computers, disconnect the power from the wireless router and then disconnect the power from the Virgin Modem. Wait 5 minutes and then reconnect the modem and wait until the lights have stopped flashing and synched. (5 Minutes) Reconnect the wireless modem and again wait a few minutes. Then turn on the Win 7 machine followed by the XP machine. This should reset the DHCP server in the wireless router.