I dunno about the mac part, but to get sharing working on your xp pc, do this....
Start -> Control Panel -> Network Connections
Right click your active connection, the one your browsing with, and select properties.
Goto the advanced tab, and check the box that starts "Allow other network users...", selecting your wireless card as the home networking connection.
Apply it all and press ok, then go back to network connections and goto your wireless cards properties page, in the "Wireless Networks" bit, press "Add" type a SSID for your network, and I would suggest using authentication of some sort too, remember to check the checkbox that starts "This is a computer-to-computer ( ad hoc ) network".
Now you have a network key and stuff to use on the mac, your pc will have the ip address 192.168.0.1 and hand out ips to other wireless devices in the perimiter no matter what os they use, so long as the authentication is correct ( if there is any )