share internet over wireless ad hoc network

How do you set up Vista to share it's (wired) internet connection with other
computers on a wireless ad hoc network?

I have a desktop computer (with Vista) connected to the internet through an
ethernet cable, and I have a wireless PCI card that I want to use to share
the internet connection. I'm trying to create a wireless ad hoc network
between the desktop and my laptop (Windows XP). I can create the wireless
network between the two computers, but I can not connect to the internet
(wirelessly through the desktop) with my laptop. On my desktop (vista) I
checked the box to allow Internet Connection Sharing, but is there something
else I have to allow through windows firewall? Thanks