Help! Need to share a dial up connection on a wireless network

I am trying to set up an ad hoc wireless network to share a dial up internet
connection but am having trouble getting the internet sharing happening.

I am using two computers:

1 - a desktop with a dial up modem and a wireless network card running on
Windows XP Home.

2 - a notebook with a wireless network card running Windows Vista Business.

I want the laptop to be able to connect wirelessly to the desktop and share
the dial up internet connection (once that is working will then want to share
printers).

I can get the computers to connect to an add hoc network but when I try to
set up the Internet Connection Sharing it doesn't work. Any help to get it
working would be greatly appreciated