I think I might have used this page for help. Google shows up a lot of other similar pages too.

Basically, my XP laptop's wireless card is used for the AdHoc network (added manually on the Wireless Networks tab, defined SSID, authentication, encryption, key(s), etc).

My laptop's wired LAN connection is then set shared (on the Advanced tab).

Firewall is switched off - only because I have a number of NAT routers between me and the real world. Maybe that's the difference between us?

Hope that helps.