Paul, there are some info missing. If you have an acsesspoint and two systems using the wireless connection, where is your internet connection you want to share? dial up? Networ/dsl? Bluetooth even?
If you can setup that internetconnection on your mac as well, you can share it for the windows laptop over the wireless lan access point. Check out System Preferences -> Sharing.
iBook 600; 12''; 640mb; 8mb Rage; DVD-CDRW-Combo, 20GB
P4 1.6; 2x80GB Raid1 (file-server)
tiBook 1Ghz, Superdrive, 768MB, 64mb 9000, 60GB