get two wireless network cards one for each pc. put the cards in the pcs. install the drivers. At this point, a icon showing a monitor with radio waves coming out of it should appear in the system tray, double click it. a list of wireless networks should appear.
if not, you'll need to tweak your network card's properties. ssid and channel should match the settings for your router. the network card should be set to infrastructure. If WEP is enabled on the router, you'll need to add the key for the network card (if wep's enabled, xp should popup a box asking for the key when you try to connect to the network)
double click a network in the list and it should connetc you to the network.