On your XP machine look at the Network Connections via Control Panel.
You should see a Wireless Network Connection - it may well have a red cross in it saying it's turned off.
Right clikck on the icon and select View Available Wireless Networks. You should see your own router and those of people nearby. Click on the one you want.
If all this fails you may have a wireless card / driver issue. There's a repair/test button you might try.
Peccavi
January 2009