1) Share your wireless connection in XP.
2) Set the IP address of the Amiga as 192.168.0.2
The ethernet port you are using to connect to the Amiga will automatically become 192.168.0.1 (It`s the M$ way, you cant change it if using ICS)
3) Set your gateway and DNS (on the Amiga) as 192.168.0.1
Job done!
(PS network mask is always 255.255.255.0 in this IP range)